Big City Greens is absolutely funnier, though, and that has to be worth something to someone. Honestly for being a more "kid-oriented" show on the surface BCG puts way more effort into its wordplay, gags, punchlines, and cultural references and the overall style of the dialogue feels more like Gravity Falls (which was cribbing most of its style from The Simpsons), while Amphibia feels more like an older 00s-styled kids' show where most of the humor is situational or slapstick.
Amphibia has world-building, lore, and stakes (and I'm liking the "road trip" angle S2 seems to be headed in) which endear it to the people who relentlessly obsess over that sort of thing, but it honestly feels comparatively less modern in its tone and pacing the majority of the time. I honestly think a normie adult stumbling onto it while their kids are watching TV is far more likely to get a chuckle and some degree out of enjoyment out of BCG vs. Amphibia, and the ratings seem to reflect that.
You can compare it to how SU may be extremely popular online, but obviously there must be a reason why TTG and Gumball get aired far more often instead. Broad episodic comedy doesn't bring in the same sort passionate fanbase and probably doesn't move merch as well, but it's more accessible to more people overall and the raw numbers are what matter to the Mouse over "social media engagement" or whatever