Why are YouTube comments repetitive in many videos?

Could be ‘bots that troll around looking for keywords on a video and dropping the same vaguely relevant comment on every one they find.
Could also be lazy humans…. copy/paste a ‘clever’ comment everywhere it will stick.

Youtube commenters are idiots and trolls.

Well, first, a lot of new American media is hyped up and is actually extremely crap, particularly compared to memorable older media. People remember the good things more than the bad things, but it’s pretty clear that there were some quite good types of media in the past that are much harder to find in the most over-hyped new media. So I see that type of remark as often a correct appreciation of good things in contrast to the focus that mainstream media gives to often-bad new media (because it wants to sell those new products).

Second, people who browse the web tend to read very little of any page they visit, which again is reasonable because there is so much low-interest content. They’re much more likely to want to say their first thought and move on, than they are to read everyone else’s comments and consider whether others have already said what they have to say, or not.

Third, YouTube comments are programmed to show people only the most recent and highest-up-voted comments. And there tend to be hundreds or thousands of comments on many videos. This adds to the amount of effort it takes to survey the comments and see what others have said.

Fourth, the people who comment on YouTube videos, especially the ones that attract much attention, tend to not be very original or sophisticated, to say the least.

Fifth, people on YouTube tend to view the comment section as a kind of gutter into which they are welcome to spew whatever remark they feel like.

Sixth, there seem to be many people who want to feel like they are part of a community, and like they get that by saying what they think the community will respond well to or agree with. So they say very predictable things that other people tend to say about things. This happens on almost all social media, and in real conversations too. They repeat things they think others will agree with, for the reward of being agreed with themselves and feeling like they are part of something.