In my experience, pretty much every single person who says/thinks that, has heard maybe a handful of Beatles songs.

That would be like seeing some normal ugly grey river rocks and saying "damn all rocks are ugly and boring" when in reality there are hundreds of unique and colorful looking rocks in existence.

The beatles cover several dozens of styles and genres within their music, the chances of not liking a single on of these styles are remarkably small, and if it rings true for you, you probably just have a shit taste for music tbh.

Incidentally it tends to be the same people who avoid/condemn entire genres of music due to their same undefined tastes in music.

I can definitely see how people who have only heard like, all you need is love, and come together, and let it be could be like "eh it's a little over rated" but they also simultaneously out themselves as haven't hearing the majority of their music.

There are very few bands that require you to listen to their entire catalogue to gain a proper opinion of, and I think that has an influence on how people feel about bands that do have this requirement. Usually you can listen to a few songs from a band and be like "eh". But with the Beatles that doesn't really work out, because their next album they might as well be an entirely different band playing an entirely different genre of music. You can't listen to a rap album, and then be like "man I guess all heavy metal sucks", and this is essentially what people do with the Beatles.
The beatles have made hip hop songs, RNB songs, rock songs, ballads, heavy (for the time) metal songs, psychedelic songs, indian sitar music, progressive music, pop music, blues music, jazz music, orchestral music, etc.etc.etc.

You simply just can't form an opinion based off of a few songs, yet most people do, and that's why people think the Beatles are over rated.