From what I know, F-Droid compiles apps from source so you can be sure that the code you’re running is actually made from the source code that it claims to be built from. On most other platforms, the developers could be uploading malicious programs that actually have the code changed from what’s shared online as its source code. Then add the fact that other developers can and do look at the code, and what changes are made from version to version.
It could just have something to do with the fact that many people think ads are not only annoying but also highly manipulative, creating artificial needs in people, a tool to make already successful and rich companies even richer, … and the surrounding technology to power them is unethical, hoarding tons of information, building profiles of people, tracking which websites they visit, what search terms they use, …
When people talk about blocking ads, being frustrated about them showing up, it’s just kind of disrespectful to be like “well you could just pay for the service, you know?”. Besides, who knows how much actually ends up in the creators’ pockets.