Oh no, this is on purpose. WhatsApp, Signal, etc. want to maintain control of the clients at all costs because of user tracking and monetization. If this wasn't the case, they would allow some form of non-smartphone access. As it is they keep their API's closed and ban anyone trying to use any form of third party tools.
In the case of Meta (WhatsApp) this is about keeping their silo-suite closed. People may hate Facebook, but if they can keep these users via the popular WhatsApp, they can keep up their numbers. What Signal really wants is anyone's guess. Something to do with that shifty cryptocoin I guess.
> I just want the freedom to use an "alternative" system like my Linux desktop or maybe some eclectic device like PinePhone or those modern PDA things or whatnot […]
Absolutely.