1. Sirens are going out of fashion for emergency warning. I don’t think there is an operational siren in my county. Air conditioning, well insulated homes, and double pane windows, have reduced the utility of sirens, which came into vogue before central heating and air conditioning, and when houses in Texas tended to be made with thin wood walls, with no insulation.
2. Even back when sirens were used, Texas is a big place and covering the entire state with warning sirens would have been a massive project.
3. What do you do when you hear a tornado siren? You grab a beer and watch for it on the front porch.
3.5 What are you supposed to do when you hear a tornado siren? Go hide in the bathroom or closet... Not sure how much that would help against flash flooding...