Because it's very, very easy to check.
You have to understand that when everyone gripes about MMA boxing being rudimentary, they somehow neglect to mention that the kick fighting is as well. I know the difference doesn't seem huge, raising your leg to check a kick vs just turning your leg slightly out to check a low-low kick - but it's actually huge.
You really don't have to shift your weight much to check that low of a kick, and because of that, you're going to be able to throw your own kick back very, very quickly. There's become this weird false narrative since like the former MMA era, that straight punches are the counter to kicks. That simply isn't true against a skilled kicker, you're going to have to kick back and get their respect. Even if you're the 'inferior' kicker, merely conceding the entire kicking game and only throwing back punches is a fool's errand.
Sure, you will see guys get caught with a punch while kicking, but I'm going to assure it is not the norm as some announcers propagate. Letting a guy kick you and only throwing back high is just an invitation sign for you to get kicked over and over.