Because scaling limits. The stuff we're already getting uses very highly doped silicon. You could go smaller or faster if you could dope it more to keep the fie

Because scaling limits. The stuff we're already getting uses very highly doped silicon. You could go smaller or faster if you could dope it more to keep the field characteristics viable, but more doping would screw up the silicon lattice. "More" also generally means an orders-of-magnitude increase. There are a few alternative processes and materials, but they're costly as all hell and hard to do in bulk.