Hmmm... the trouble is an amplifier or "smart" antenna cannot receive and decode 2 x datastreams at once, which is what the diversity tuner does. A diversity tuner compares the two signals and measures their error rates and selects the best signal at any time. This can mean a rapid switching between tuners and signals at multiple times a second to provide the stable output signal.

Really, in a mobile environment, the dual-antenna (with antenna diversity) and dual-tuner is the minimum required to get a stable signal.

The modern BMW automotive digital TV tuners actually have 4 x tuners and 3 x antennas:
1 tuner to scan and find signals
1 tuner to decode the signals and find out what channel is where
2 tuner to act as a diversity function for the currently tuned channel
3 x antennas which the RF antenna amplifier constantly switches between based on the instructions from the currently tuned channel.

So you see it is a compley receive environment requiring a lot of effort to get a stable, reliable signal

I expect that ATSC automotive digital TV tuners will appear, complete with diversity antennas and diversity tuners, but it will take time for the market and demand to develop.