Eh, the right can have their dumb memes too, I say. I've seen dumber memes than that, honestly.
This is why they like it: It's an inside joke, in a way. Memes are a way of identifying oneself culturally by being in on it. Let's go Brandon was a moment of either absurdly dumb mishearing of a chant or an absurdly dumb intentional reframing of a chant. Either way, it signifies insider-ness, pokes fun of the president, and lightly skewers the "mainstream media" all at once. That is appealing to a certain group of people.
Us not getting the "joke", such as it is, might be part of the problem with our cultural division. We spend so much time speaking to our echo chambers, riffing on the latest nonsense over and over until we don't hear anything else. Social media doesn't allow for much of anything else, if that is our method of discourse.