To expand on the above, factory drilled tanks are drilled first, then the glass on the bottom is tempered.
MOST standard tanks are not drillable on the bottom, just the back and sides. Most have a sticker somewhere in the bottom trim that will say "tempered, do not drill."
I don't think drilled tanks will ever become the standard, while higher end freshwater systems are frequently done with "reef-ready" tanks, the overwhelming majority of freshwater tanks are still done with hang-on-back or canister filtration. And the majority of aquariums in general, are freshwater.
There are way more choices in factory-drilled aquariums than there used to be, that's for sure.
Jenn