In the specific case of leaving the runway on a high-speed exit designed for this, since you are already slowing down, you may enter the taxiway at 50kts as you are already slowing down (low thrust, brakes already active, pilot mindset oriented towards stopping). The taxiway will also be designed so you won't have to stop suddenly just as you leave, either.

Accelerating to 50kts puts you in a much worse position to stop: higher thrust (which does not disappear immediately the thrust lever is retarded, turbine engines take time to slow down as well as speed up), brakes not applied, pilot mindset not oriented towards stopping.

Should the aircraft have to stop when taxiing, it is going to be far less pleasant for those in the cabin at a higher speed, too - and the likely stop distance for the aircraft on general taxiways is designed for the lower taxi speed, not a higher speed.

Additionally prolonged taxiing at high speed causes tyre sidewall heating and, in the worst case, tyre failure, so time at high ground speed is restricted to takeoff and landing, not going around and around on the ground.

The topic of "Ryanair taxies too fast" often comes up on PPrune and related places, with the usual response of "The flight ops department will find out and there'll be serious trouble if we exceed 30kts" and "We're not cowboys" and so on. In summary, they may taxi fast, but not too fast.