I know it's autism and a stretch, and the appearance of them in Break it Boris was leaning way more toward comedic/general fanservice. But put yourself in the shoes of the money vacuum that is Disney. You've got a tried and true model with the MCU, learned a brutal lesson in overexposure for cinematic universes with Solo, and have a public that isn't really clamoring for a princess film every year but probably wouldn't mind an uptick in their appearance. And when this said public consists of little girls ripe for merch sales, the general public who just goes to see the latest Disney films for their consistent quality and a nice family weekend, and droves of tumblrites who are as ready to overreact to a trailer for a princess film as the average rebbit-abusing YouTube reactor for a Star Wars or Marvel flick, you mean you AREN'T gonna take that opportunity? Especially when all those "IS LE DIDNEY MOOVIES ALL IN LE SAME UNIVERSE!!!11" theories have been all over the internet for fucking ages and normies always parrot it online ad nauseam.
Of course they don't NEED to do it, especially when these films make a billion each on their own anyway. But I feel like it'd be untrue to say the mouse hasn't tossed around the idea at all.