I think one of the biggest problems isn't that those dense MCs exist, but that the denseness is often more the result of the story instead of the character.
e.g. Ichika from IS. I don't really have a problem with him being dense and not reacting to the girls advances, because it's made clear he sees them more as friends and less as women. But relatively early in the story, he actually discovers his feelings for Houki, wants to kiss her, is interrupted... and he doesn't do anything else for the next volumes. Bask to status quo.
At this point he isn't dense because of his character but dense, because the publisher still wants to tease the harem ending and sell more volumes and merch of the other girls. That's the moment a dense charactet becomes detrimental to the story.
A more positive outlook for a dense character would be Shiroe from Log Horizon. He has a reason for his dense behaviour (relatively few RL contacts), he shows the same problems in fields outside of romance, and in the end he develops, acknowledges his feelings and takes the next step. In core, both characters are dense, but one is driven by the character while the other is driven by the publisher.
And especially JP webnovels and light novels have a big problem with characters who are dense because the publisher wants to print three more volumes. Most of those harem MCs who'll pick the obvious girl at the end but because there isn't any good story besides girls drama, he has to be stupid so that he doesn't decide. Same for a lot of romance stories, who traditionally end with the first kiss, so they have to be stupid and never see the signs, so that a story worth 5 volumes can be printed over 10 with an underwhelming ending...
And I think that's why people got so tired of dense MCs and calling them beta. Because they acted that way without character or story reason. They just did to give unnecessary drama. And when people found chinese and korean web novels, who are often the extrem opposite, they despised those characters even more. So much, that "beta" and "dense" are more of a meme word without any meaning...