Why do instances not run ads?

It probably goes against the philosophy or whatever of FOSS or Lemmy itself, but why not be a little evil so that you can actually sustain yourself? Donations can bring us far, but small non-intrusive ads can be a bliss in the skies for the people actually hosting the instance. Especially if there are millions of users uploading thousands of images and videos. This is extremely expensive.

Is running ads really that taboo?

EDIT: some people seem not to get the point of "millions of users", which presumably includes non-techies that do not use adblockers. I mean that without ads (or mining?), no instance would be able to scale to the point where it can compete with Reddit for example. If you were to want that. And not for profit, but solely for sustainability.