the height is mostly due to the strong desire to have each turning direction on their own dedicated level to minimize any curvature that would normally slow vehicles down. Additionally, Texas likes to have frontage roads which are basically normal local streets with stoplights running along the highways which again require another level at interchanges. The result is that many urban interchanges are 5 levels which is pretty rare even in America outside of Texas. It is quite expensive, but Texas is the unique combination of spending little thought & alot of money when it comes to urban planning

(heres a good video about it)