If demand for home is so high, why aren't more people building?

Forgive my ignorance here-- I read about the housing market like most of us do and one thing I see repeated often is the shortage of single family houses in the country. Experts say we're around 8 million homes short of what we "need."

From my basic economics lessons I read that supply usually rises to meet demand at whatever price point is sensible, yet that doesn't seem to be happening. So I'm curious what's causing this:

1 Is the home building and selling process just too flaky and drawn out to where builders, sellers, and buyers fail often enough to where it's just not worth it?

2 Is the home shortage people talk about actually just a home shortage where people want to live for cheaper than is realistic (I'm in Dallas and there are probably hundreds available for cheap 40 minutes outside of town and very few available in the city).

3 Are people's expectations for what a house does / should cost just out of whack and they're not actually financially prepared to buy one?

4 Something else?