Forks on bikes and motorcycles do pay either a weight or stiffness/strength penalty for their lack of triangulation.
One can imagine systems for steering the front hub with a linkage rather than a conventional cantilevered fork.
That sounds more heavy and complex than it's worth for a bicycle once you have it able to handle the full range of motion needed, but that's just a guess, and it's not say there's not some other application for such an idea.
One piece of the answer is that the main drawback to a cantilevered fork is that they're vulnerable to being smashed into things head-on.
If all else was equal that would be a nice vulnerability to eliminate, but it's hard to imagine a situation where all else is equal, especially considering the forces the bike encounters in such incidents are quite high.
Good forks offer a degree of valuable micro suspension even if they're not suspension forks, and losing that just to be able to smash into things would not be a welcome compromise for most.
This is to say that a more triangulated design employed in a way that increases stiffness likely would be unwanted.
If you could keep it at the same stiffness but with less weight, I would say the main reason that doesn't exist is no one's been able to figure out how.
bicycles.stackexchange.com/users/25959/Nathan Knutson
If the fork was also welded to the frame at the bottom, you would not be able to steer.
There are still two points of contact to the headset, the down tube and the top tube.
Also, the fork needs to be compliant and not totally stiff for good riding properties, otherwise the ride is too harsh and braking could also be bad.
In the back it works differently.
One sort of drags the rear wheel behind and it reacts to bumps and other forces differently.
Also the braking is very different.
And one does not steer the rear wheel.
bicycles.stackexchange.com/users/21133/Vladimir F
A triangle is a truss.
Old bikes had trusses front and back.
Two (seat tube - chain stay - seat stay), and one (seat tube, down tube top tube).
As bikes and materials and construction methods have evolved manufacturers were able to develop designs that are not trusses - either by beefing up a single tube or some other geometry.
Suspension systems have required unique designs between manufacturers.
But a truss is a classic construction component and is the simplest way to add strength and stiffness to a frame.
bicycles.stackexchange.com/users/49921/boodoodoo
As others have said there is basically no good way to add additional structure to the front wheel to triangulate the support for it without lots of extra weight and complications to allow it to turn and steer the bike.
It turns out we can make forks that are good enough without triangulation.
Also bear in mind that the rear part of a bike frame has to transmit forces from the crank though the chain to the rear wheel, which which means it has to be stiffer.
bicycles.stackexchange.com/users/24228/Argenti Apparatus
They are not necessarily stiffer in the back merely because that section more closely resembles a triangle.
The geometry and structure of a bicycle is more complex than that.
The primary purpose of a bike frame is to hold your weight over the wheels, stiffness isn't the main goal.
A relative amount of stiffness is necessary, but a little flex is good too.
Part of the purpose of putting rake into the fork is to allow for flex.
Modern mountain bike frames aren't stiff at all, and have pivots that allow for many inches of flex.
I have a carbon fiber cyclocross frame that is about the stiffest thing I have ever ridden and that doesn't make for the most pleasurable ride.
bicycles.stackexchange.com/users/47206/bradly
I know that triangulation of structures makes them stiff. Why are bicycles stiff at the back but not at the front?
Why are bicycles triangulated at the back but not at the front?
bicycles.stackexchange.com
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bicycles.stackexchange.com/questions/68632
bicycles.stackexchange.com/users/35047/veronika
There is basically no good way to add additional structure to the front wheel to triangulate the support for it without lots of extra weight and complications to allow it to turn and steer the bike. Also it should be borne in mind that the rear part of a bike frame has to transmit forces from the crank though the chain to the rear wheel, which means it has to be stiffer.  There is some disagreement on this though as some bikes, particularly MTB are not stiff at all. In fact, the primary purpose of the bike frame is to hold the rider's weight over the wheels. The considerations are a lot more than just stiffness, and the front steering and different forces make different front and rear structures more useful. Modern bikes are better at providing the right amount of flex without using triangulation in many places, and the support at the down tube and top tube is sufficient for strength requirements. Bicycles are not normally meant to be as stiff as possible. Modern mountain bikes have suspension that allows a lot of movement, and the rake of a fork on most bikes is there to allow some flex.
The considerations are a lot more than just stiffness, and the front steering and different forces make different front and rear structures more useful. Modern bikes are better at providing the right amount of flex without using triangulation in many places, and the support at the down tube and top tube is sufficient for strength requirements. Bicycles are not normally meant to be as stiff as possible. Modern mountain bikes have suspension that allows a lot of movement, and the rake of a fork on most bikes is there to allow some flex.