Then why are rents going up drastically all over texas, even in small towns where damn near anything can be built?

That may be one cause, in some towns and cities, but it is far from universal. For one thing, demand growth has accelerated in many places, so building homes at a historic rate went from sufficient to meet to demand, to insufficient. You have flippers taking low-priced homes off the market, and you have AirBnb'ers taking other homes off the market (or not selling theirs when they move). There are higher expectations of what a house should contain (sq ft, # of bathrooms, closet/garage size, quality of fixtures) so lots of housing stock technically exists but is not really part of the supply for most buyers.

The root cause is that houses are built for decades. It's impossible to plan ahead at this timescale. The alternative is to make standard places for stackable houses, with flexible infrastructure (electricity, water, sewer, road, parking space) to fulfil demand in days, instead of decades.