So now you have the same number of pixels as from that one big monitor, but you either need a fancy mounting mechanism for putting the monitors above one another or else need twice the width on your desk. And you get a big wide thing you probably can't see all of at once, instead of something a more natural shape. And it's divided into four bits which limits the possible shapes and sizes of your windows. And it's more expensive.

Again, for sure you might have good reasons to choose four smaller monitors instead of one really big one. But (1) the one big one has definite advantages and (2) I repeat, I wasn't saying "hey, everyone should get one of these things" but "yes, as it happens such things do exist and here's an example".