Garages are expensive to build and if you design your roads so that no person with the means would choose anything but driving and ignore the rest, then sprawl becomes the reasonable choice, since people are driving to every destination anyway. What's the point of the added expense of a garage if you don't really care about being close to other things nearby?

In addition to what others have said, parking lots are also an easy and cheap way to keep vacant land for speculative purposes.

Basically, our current property tax system allows you to buy a vacant lot, sit on it for a number of years while paying pennies in taxes (because a vacant lot or a parking lot is very low value, even if the land is very high value), and then resell for a much higher price after the city has grown around it. Basically no work, but tons of free profit.

The remedy? Tax land instead. That way, your tax burden is based on the value of your land, not the value of your improvements, so that this form of land speculation becomes uneconomical, while also strongly incentivizing you to develop something more valuable, e.g., housing, offices, etc.

Not much underground because Houston is basically a swamp.

Nothing up high because why do that when there’s a whole bunch of flat ground?

Never mind the lack of public transport, bike infrastructure, or the general fact that literally everything is built way to far apart to make walking an option…

Houston is built on a swamp so you can't really dig a lot without dealing with humidity, water, etc. Multilevel means investment and debt. It can take many years to break even. You can make $25/hr per space and quickly make profit and sell in 10 years for 10 times what you paid for. There are multi level parking though.

Basically, it's greed.

One reason is it is millions of dollars more expensive to build underground parking.

I think I’ve seen examples where the underground parking for a commercial building costs as much as or more than the building above.

There is underground parking in downtown Houston as well, but during heavy rain they flood like crazy. I want to say the underground garage near the theatre district was closed for a few months after Harvey.