Short answer: we drive everywhere.
More detail: The key takeaway is that the cities are really spread out with people commuting in all sorts of directions that make it really difficult to identify good urban-area-wide lines. And it'll all be very expensive. So we drive. A lot. But there are occasions where rail is useful in Dallas and Houston.
Here in Austin, we have a slow bus system combined with a somewhat bizarre commuter rail line that goes from one particular suburb to one particular area of downtown, and is useful for a very small number of commuters who happen to live in that one area of that suburb and work in that one area of downtown. And it took ten years of political fighting to get that. Austin voters just rejected a more comprehensive urban rail plan for reasons that are, frankly, kind of understandable. The city is just kind of scattered all over the place, with people living and working everywhere; from what I can tell, it's a bit hard to identify useful lines and it would all cost a ton of money. Meanwhile, city leaders spent twenty years pretending that Austin wasn't growing while it was actually exploding, so our highways are bursting and there's no money to improve them, as the gas tax is woefully low and the State has prioritized Dallas and Houston. I'm able to get to some places by walking from my apartment (most notably, of course, my university), but I drive a fair amount.
San Antonio is about the same as Austin, except they don't have any rail at all. Driving in San Antonio is maddening.
Dallas/Fort Worth has probably the most well-developed rail network in the state, but it obviously pales in comparison to even the shittiest rail networks in most other metro areas of comparable size (say SEPTA or the BART/Muni clusterfuck). But people still drive a lot, enabled by the fact that D/FW actually has a damn good freeway network with reasonable traffic. And the place is huge; the D/FW Metroplex is larger by land area than Connecticut and Rhode Island combined.
Houston is a mess of sprawl with psychotic drivers who always do 30 mph above the speed limit when there's no traffic. But their traffic is pretty bad during rush hour, even with
freeways that have 26 lanes. They are working on a rail network that's improving to the point of usefulness if you live in the inner loop. I'll probably be moving to Houston after I graduate from UT in May. My office is downtown and I plan to live in an inner suburb with a direct rail connection to right by my office, so that will actually be useful. But driving still happens a lot. Unfortunately, the trains all run at-grade, which causes the maddening problem of cars running into the trains quite often.