Can I get a bright rear light using AA batteries, or is there a compelling reason why nonreplaceable batteries are so widespread? TL;DR: I like the convenience and the ecology of using AA/AAA batteries in a rear light. Yet the rear lights using AA batteries I see on the market have puny power. Does anyone build somewhat powerful (30+ lumens, say) rear lights that use AA batteries? I am quite happy if they are drained by flashing (at 30-50 lumens) after just 2-3 hours. I can carry spares. In any case my usual ride is two hours long.
A bit of context: In my jurisdiction reflectors are mandatory, but lights are optional. Since I ride mainly soon after sunrise or in the two hours before sunset, during the summer only, and recreationally only, I intended to skip headlights and rearlights altogether. I avoid main roads and manage to ride on trails, with the occasional stretch on side roads. I miscalculated routes recently and needed to ride just 300m on a road with some level of traffic. A car screeched to a stop behind me, presumably because the driver was distracted by their phone. I'm now revisiting the decision to not have lights, since flashing front and rear lights may increase the safety of my riding, on both trails and side/main roads. I prefer to use replaceable batteries (see below), yet I'm wondering if the much more powerful lights in nonreplaceable lights, especially rear lights, would reduce the chance that a distracted driver would miss seeing me. (The aforementioned 300m stretch was uphill, and I was struggling to keep up with anywhere remotely close to the speed of the cars on that road, 50-60km/h, and so I counted more like an obstacle at the edge of the pavement.)
An ongoing fad is to switch all consumer devices to custom-sized, non-serviceable, permanently attached Lithium-ion batteries. Yet if removable, rechargeable AA batteries had appeared on the market as a new invention after consumers were accustomed to built-in batteries, the replaceable batteries would have likely been deemed a great progress forward.

One can have an unlimited supply of readily charged batteries to fit in any one of several devices.
One can travel with a handful of batteries and never need to pack a charger, or need to be in the proximity of electricity.

To cite a non-cycling example, the Apple Wireless Keyboard, in which a pair of 1000mAh rechargeable batteries last me eight weeks of ten hours daily use, is more versatile than the Magic Keyboard, for the reasons just mentioned.
Hence with bike (front and rear) lights, I suspect that it's an advantage to seek lights with replaceable, standard-size batteries.
The abundance of custom-sized, non-removable, non-serviceable, Lithium-ion batteries in front and rear lights puzzle me. Is it just more lumens? If I intend to ride only during (the plentiful in summer) daylight hours, what is the advantage of non-removable batteries in lights?
P.S.1: It's of course a huge waste to use discardable—alkaline, ..—batteries. We're comparing here rechargeable fixed with rechargeable replaceable.)
P.S.2: (At the risk of venturing into preaching,) the ecological cost of each one of us buying headphones, lawn mowers, leaf and snow blowers, wireless keyboards, drills, screwdrivers, and cycling lamps with nonremovable batteries is enormous. The volume or weight lost in cycling components is not nearly as bad as that in larger equipment. From the point of view of the manufacturers, there could be one of many advantages. There we can only speculate. Hence this question asks the consumers of one particular product. In case you have studied this question and found an answer, what is the advantage of non-removable batteries (that may or may not be properly recycled at the end of their life)?
Most modern lights use rechargeable lithium ion batteries. Those come in three form factors: Pouch cells, cylindrical cells and coin cells.
Pouch cells come in a lot of different dimensions which gives manufacturers a lot of flexibility when designing the product. They also have cables directly attached and are quite fragile. Good for weight and size but bad for handling by a user.
Cylindrical cells like 18650 have a relatively robust metal shell. Sometimes they are user replaceable but somehow this never caught on. Most of the time they are directly soldered or combined into a battery pack (most older laptop batteries are just a plastic shell and some electronics around 18650 cells).
Coin cells are usually directly soldered to the circuit board. I guess you could make a coin cell holder but that just increases size and weight and introduces another point of failure.
So only the cylindrical cells could really be replaced by a normal end user.
Another factor is charging current and voltage. Lithium ion batteries differ slightly in charging cutoff voltage and charging current. Some can be charged up to 4.3V, others only up to 4.1V. Charging a 4.1V cell up to 4.3V can cause it to go up in flames. So if the device has a built-in charger you’d somehow have to make sure that a cell with the correct charging cutoff voltage is inserted or that the correct voltage is selected. I guess yet another factor is that a battery charge level indicator can be directly integrated into the device’s circuit board instead of being a separate circuit inside the battery. Charge level indicators either estimate the level via battery voltage (which again, varies slightly between LiIon batteries) or by measuring the energy going in and out of the battery (Coulomb counters)
Apart from that there are generally some advantages in not having a hatch and battery holder for user-replaceable batteries.

Less complexity (more robust, cheaper)
Easier to make dust and waterproof
Smaller
Lighter
More flexibility in the case design
Product only lasts about 5 to 10 years. Of course that’s only an advantage for the manufacturer because they can sell you a new one.

There is simply no easy way to make a tiny, water-resistant, 40g light like this with a 420mAh Li-Ion battery and have it user-replaceable: