Because we paid to not have to put up with this garbage. There's so many better ways to do this - look at nzb360 - https://nzb360.com/

They added a new/better interface you have to pay money to unlock. When they add new features/services you now have to pay to unlock. What you paid for originally, still yours. Want to get access to the new stuff? You can either pay a subscription for "everything" or pay one-time-unlocks for features.

Then I look at serviecs like lichess where they just operate 100% on donations and users helping by adding their devices into the pool of compute for analysis.

"Shove ads in" is the low, easiest, tackiest way to "annoy" your users into paying. Those that already paid once are annoyed the goalposts have changed. Make the app worth paying an upgrade for, don't just go "well it's still shit but now there's ads unless you pay!"