The issue with transistors is that they now exist at the scale of a few nanometers in size?"only a few silicon atoms thick. They can't practically be made any smaller than they are now. If they get any smaller, the electrical current flowing through the transistor easily leaks out into other components nearby or deforms the transistor due to heat, rendering it useless. You need a minimum number of atoms to make the transistor work and we've functionally reached that limit.