Why do anti-ice systems on airplanes with automatic ice detectors still need to be manually selected on? As even minuscule amounts of ice accumulation on aircraft can cause tremendous loss of lift, increases in drag, decreases in engine thrust, and/or flight-control impairment, some method of detecting ice is essential for safe flight, even for aircraft not certified for flight into icing conditions.  Various methods exist for doing this, ranging from examination with the Mk. I eyeball (for many light general-aviation craft, and even some larger planes) to sophisticated electromechanical systems utilising vibrating probes which change frequency when made more massive by ice accumulation and set off an alarm in the cockpit.
For aircraft not certified or equipped for flight in icing conditions, the detection of ice lets the flightcrew know that they need to get out of the ice ASAP.  For aircraft which can fly in ice, it signals the need to activate the aircraft’s built-in deice/anti-ice systems; these come in three main varieties (melting the ice using hot engine bleed air, melting it with resistance heating from electrical heating elements, and mechanically breaking it off with pneumatically-operated deicing boots), but all of these still need to be manually turned on by the pilots, even for aircraft that can automatically detect icing.
Given the potentially-catastrophic consequences of allowing ice to accumulate unchecked on an airframe, and the ease with which a flightcrew (especially if fatigued or already task-saturated) could overlook or forget about an ice warning, why don’t aircraft with automatic ice detectors and built-in anti-icing link the two together, and have the ice detector automatically turn on the anti-ice system when it detects icing?
Because most of the time the system has to be on even when there isn't detectable ice accumulation.  Because of what I like call &quot;technical icing conditions&quot; but which are just called &quot;icing conditions&quot;:
ICING CONDITIONS:

TAT (total air temperature - accounts for temperature rise from compresibilty) less than 10C

Visible moisture (cloud, haze, anything that is water)

SAT above -40C


You also have to have WAI on for takeoffs when OAT is less than +5C with visible moisture (including visibility under 1 mile) regardless of whether there is actual ice formation (water streaming back on the wing on takeoff can freeze upon rotation due to the temperature drop from the low pressure over the wing).
When you get the wings deiced, you still have to have the WAI on at the start of the takeoff roll if departing in technical (or actual) icing.  You don't want to cook the thick type IV holdover fluid, so you leave WAI on selection the very last thing as you pull out to line up for the TO roll (the CRJ200 has an evaporative WAI system which meant the leading edges get VERY hot, about 105C, which could cook the fluid and make it crusty).
When flying, the only time you can leave the WAI off when in technical icing conditions is
above some speed (on the CRJs it was 230kt), unless you get an ICE caution from the ice detector.
The result is a system that automatically turns the WAI on when ice is detected would only cover about, oh, maybe 10 - 20% of the time you are legally required to have WAI on.
Cowls are similar, but have to be on even more often because there isn't the 230kt threshold for actual accumulations only. They have to be on all the time you are on the ground less than +10C with visible moisture (freezing around cowl inlets from velocity temperature drop).
Without the 230kt exclusion the wings have, you have to have the cowls on pretty much all the time you are in cloud, actual icing or not, so you are turning them on and off quite a lot if climbing/descending through layers (some pilots may just leave them on), whereas outside arrivals and departures the wings are only on when you get actual ice.
So you could automate the system like that, if you could figure out how to make the system reliably detect &quot;visible moisture&quot; outside somehow (probably possible, but very expensive).   Otherwise, the decision to turn it on is a crew decision based on the view outside, probably 80% of the time.