There’s a few clients for Signal, nobody is preventing developers from creating apps; there’s Molly, gurk-rs, Axolotl, Flare, signal-cli, Pidgin (with the Signal plugin.
The problem is 3rd party clients don’t implement all features because it takes a lot of work and they’re created/developed by volunteers - just take a look at Matrix and how many clients support all features or even just group end-to-end encryption (E2EE). Last I checked many third party Matrix clients didn’t support encrypted group messages, primarily just Element, the reference client built by the matrix developers. So you have the same problem on Signal that you have on Matrix.