I have a little over half a million karma, about 200k from posting links. I think it's incorrect to suggest that the "vast majority" of content is manipulated. You see the same names because they understand the game of karmawhoring. If you have one link which can fit in multiple subreddits and your goal is to get as many points as possible, you post it to all the relevant subreddits, repeat for every other link you have. It's a numbers game. More posts means it's more likely for one to stick, and you get better at it with time. You eventually pick up on what times of day are most effective or which subreddits get more consistent results, things consumers of the content wouldn't ever know, so they try to do the same and their post flops and they say that clearly it's manipulation.

There is some manipulation, especially if you consider mods who remove posts at their discretion, but it's far from a "vast majority."

I do have concerns related to some political subreddits, where there is evidence of bad actors attempting to influence the 2020 election. A subreddit (whose name escapes me) just had all of its moderators removed because it ended up being one person running three or four dozen alternate accounts.