We didn't do "vehicle" levels because 1. we ran outta time, and 2. LEC had other products that would perform the same function, so they didn't want to be basically 'competing' with their own products.
We had some vehicles in mind, namely a drivable STAP and AAT, plus an underwater sub level after Otoh Gunga, but all of those got axed in December of 1998 when we saw that we would have no tuning time. At that point, it became easier to simply focus on the "human" player aspect than making a playable "vehicle".
Add to that, Ep1 Racer would be hitting the shelves the same day as TPM, so if we actually included some podracing sections, it might lessen the demand for Racer. Some people would buy TPM and play it all the way through before buying Racer, and if TPM had racing action, people might think that "well, there's this podracing game, but I played some of it already." If anything, download the Racer demo, play TPM up until the podrace scene, save the game, exit, play the Demo and then continue ...LOL.
As for why there was no space combat, well, you got me. Same reasons, basically, but I agree that it would have been nice. But not a lot of people would want to play Anakin