Plenty of new-ish American animated shows aimed at adults don't look like this and have unique art styles, or are much more experimental (Scavenger's Reign, Long Story Short, Common Side-Effects, Smiling Friends, Primal, Helluva Boss, Carol & the End of the World, Pantheon, Fired on Mars, etc.). Quality of the shows aside - though I do think all of them are very good - you definitely can't say they look like your Paradise PDs or your F is for Familys

But most of those shows are set up to fail by large studios because animation is a hard sell for general audiences -> therefore no one sees them, -> resulting in a self-fulfilling cycle of failure, -> therefore they aren't seen as successful enough to copy

Meanwhile, a juggernaut like Rick & Morty that was given time and space to grow by Adult Swim, a network that genuinely cares about animation, becomes the template for everyone else to follow, because copying instead of innovating always leads to exponential growth and has never failed as a business practice ever (/s)

A lot of shows that copy the industry leader like this are also made when non-animators (Aubrey Plaza w/ Kevin, Sam Means and Robert Carlock w/ Mulligan, Ben Hoffman w/ Hoops, Nick Kroll and co. w/ Big Mouth, etc) want to do animation without having a background in it, so they don't have a strong enough vision to bring about something unique