In theory you are correct - there is more potential power available in a decent set of rim brakes than a hub mounted disc brake due to the longer braking track providing more friction and more mass for cooling.

This is why Maguras hydraulic rim brakes were so popular in the late 90's and early 2000's - disc brake technology hasn't really matured at that point, and hydraulic rim brakes gave advantages in modulation and power over cable actuated v brakes.

I more recent years mountain bikes have switched over almost entirely to disc brakes (either cable or hydraulic), and road cycling is starting to convert because while rim brakes have some advantages in theory, in practise these advantages are very small and unnecessary, and the hub mounted disc format provides some other big benefits.

In terms of raw power, it just isn't necessary in most cases. Even a fairly basic v type cable rim brake can lock a wheel completely solid with little effort, so what we are looking for is modulation and control - the ability to fine tune how hard the brakes are applied, which hydraulic systems give better control over (as the fluids won't compress as you pull the lever, while cables can stretch).

Looking at the components themselves, a solid disc means the pads push against something completely solid and incompressible, while a wheel rim means they are pushing against a hollow section which can flex under pressure. Similarly the disc brakes caliper is one solid piece, while a rim brake has arms mounted to opposing fork legs or seat stays which have some ability to flex.

And then there are the big practical benefits that are nothing to do with stopping power - moving the disc away from the rim gives you far, far greater mud clearance, and stops your brakes becoming useless after one muddy puddle. The same also means that slight damage to a rim will no longer jam a brake (and the brake is much better protected from damage itself). Separately you also gain the ability to design rims without any need for a braking track - so mountain bike rims can be made stronger, and road rims can be made more aerodynamic, and without any worry or concern over having to form a braking track in carbon fibre (which generally sucks).

So while there are some theoretical benefits to run brakes, they are vastly offset by the very real benefits of discs in most use cases. They have all of the power and modulation we need (noting that a lot of riders even use smaller 160-180mm rotors over the 200mm+ of the big gravity riders), with other benefits thrown in.