https://f-droid.org/en/packages/org.quantumbadger.redreader/
This means shitty native apps, walled gardens and DRM.
This was the case before ad-blocking was popular.
Native apps with spyware and malware were the norm before web apps became a viable way to ensure people couldn't copy software.
DRM was around before most people had internet access, for example, CSS on DVDs was introduced in 1996[1].
For a lot of people, their first introduction the internet was through a walled garden[1].
[1] https://en.wikipedia.org/wiki/Content_Scramble_System
[2] AOL's 'Walled Garden' (2000): https://www.wsj.com/articles/SB968104011203980910
I don't believe the ads in the latter category are pulled from an ad server, they're served from the same source as real content. Could be wrong though, I haven't used Reddit in a while.