Yes there is a reason, users often don't expect to pay for services that show ads, and users who pay for services often don't expect to see ads. That's why most popular online services don't mix the two, eg. Facebook, Spotify, YouTube, Netflix, Crunchyroll, Google, etc.

"mental short circuit" better describes your argument that jumps from "it's possible to both show ads and charge user fees" to "they always will".