Because an advertiser will pay more and most people don't want to pay anything? There's a technical problem of handling micropayments but those are a lot smaller than the social/business problems: think about what the experience would be like clicking around with “Please deposit $0.25 to continue” prompts, and whether sites would bother with that when Google will give them a check every month for less work.

Google has tried this with Contributor back in 2015, and then again in 2017, but it wasn't popular because you were committing to pay a noticeable amount of money every month (which is more than many people think they can/should pay) and the people who would pay that would only pay that if they didn't see ads at all (since you were effectively bidding for your own ads, this wasn't a given). If, as is common, you hit sites which weren't using Google's ad network, you'd still see ads and the prospect of paying more to buy those out wasn't going to be a fun sale.

Now, maybe Google isn't trustworthy here but they're far from the only party to try this and there are also services like Apple News or Blendle which are trying a more targeted approach. I don't know how their sales look but the fact that it hasn't been newsworthy suggests that the status quo isn't going anywhere quickly.