For at least months in my city, the screens in buses have been showing a "broken image" icon as part of their ad cycle. I'd much rather know about the next stop.
Ads make money, but not that much. I'm not convinced they are enough to be worth the bother. To have ads you need to pay people in sales to sell them, people to install them (now that this is electronic it is easier than the old paper days, but you need a more expensive tech person to keep them working). At best they are 5% of your gross budget, so not very significant and they often hard harmful to your riders.
Transit in the US seems to be constantly on a starvation diet budgetarily speaking so I'm not shocked small sums of money get chosen over options that would be better for their users.
The MTA is a huge outlier but still gets less money for upgrades and upkeep than they need. They limped along for years on extremely outdated train tracking infrastructure and are just getting around to doing updates that needed to be started two decades ago.
What often happen is a a company comes into install and maintain the displays for "free", on the proviso that they can display ads and keep most/all of the ad money.
5% of a transit authority's budget would be a lot of money! Advertising made £158 million for Transport for London in 2019, much less than 5% of their budget.
