The first person would, most of the time, not bother making the drive, which would have the same mitigating effect on COVID transmission.

The real reason for why domestic flights don't require negative tests is because it's politically inconvenient for the government to ask for them, and because it would gut airline revenue. "Controlling the spread" plays second fiddle to matters of political expediency and money.

That, and it's much easier to impose restrictions on foreigners, even if, as of January 2021, those restrictions don't accomplish much.