To be fair texting is remarkably low bandwidth. In its original implementation it was essentially "hey here's this overhead in the cellular system that we can't really do anything else useful with, what if we stuff some characters here?" -- but I also don't think it's a critical feature, that UA has any responsibility for providing it, etc. The providers of the ground-to-air/air-to-ground infrastructure have to be paid for their services and the traffic they handle. Other airlines (or perhaps even carriers, e.g. I wouldn't at all be surprised if T-Mobile or whatever is paying a "sponsorship" to whichever airlines does that or to the ATG provider and for whatever reasons UA/UA's providers haven't or can't sought such partnerships.

It is amazing, though, compared to 10-15 years ago how far technology has come -- remember Connexion by Boeing first launched 2001 (commercial launch/demonstration 2003) but then died off by 2006 due to lack of interest. Now we're in an era where we can reasonably expect (for a cost) connectivity over vast swaths of the world... In 2014 I remember posting to Facebook from a BusinessFirst seat somewhere over the vast Pacific between Melbourne and Los Angeles ... not long after proposing to my (now) wife. Prior to COVID airlines were talking about making onboard wifi free to all passengers -- I'm sure the pandemic has pushed things back but "be patient" -- I'm sure aggressive compression will be a big part of the interim solution.