As you can see baseball is so popular within America and a lot of people engage in that sport, so there are always players with poor vision. And it would be inconvenient for them to wear framed glasses since they are fragile and could cause dangers for their eyes. Therefore, contacts became their alternative and they proved to be quite useful indeed. Some players really wear them, or they have got vision surgery.
Basically they should wear contacts while playing if they are nearsighted.
If wear framed glasses, when they look at the baseballs, there shall be based compared with actual baseball position. This shall be great danger when players wrongly missed the final position. But wearing contacts will not cause such problem and it can help tacking the ball clearer and without no mistake.