If Waymo ever raises prices drivers can easily re-join the market and compete, the cost of doing so is basically free, so how is this a monopoly? There is no barrier to entry for their competition in the food delivery market

It is right to be skeptical, but I dont see how monopoly plays into this at all. Even if we assume Google is a monopoly company with Search or Ads, how Waymo delivers has got nothing to do with Search and Ads.

Perhaps you were trying to Google is a big tech company and they have gobs of cash, and that's why they are doing it. Precisely, and it is a public market company - so if it isnt a good use of their money, people will vote with their wallet.

Also, there are other richer companies (Apple etc) who can do exactly this thing. Nobody is stopping or unfairly being affected due to Waymo delivering food.