What is it about the gearing that limits human generated power?

I have seen a few different setups with a generator hooked up to a mechanical system like a stationary bike or a bike in a rack, etc. Whether the bike tire is pressed against the generator or a large flywheel was used the power output was limited to about 150 – 200 WATTS.

Taken the human component out of the equation, (I know that we have a biological limit as to how much horsepower we can transfer into the front end of the system), what limits the power output?

Increasing the RPM of the generator will increase the power output so why can’t the pedal system be geared up for higher RPM at the generator? (Were assuming that the generator has the capacity to generate more power than currently achieved).

There must be a logical reason that I just am not grasping at the moment otherwise someone would have already built this higher power pedal system.

The only thing I could think of is this is an issue of torque.

Having read many of the answers to dynamo questions I know there are some really helpful smart people out there and quite a few funny smart-alecs. Thank you to everyone who responds whether they are the former or the later.

Now for some coffee.