it is a development machine with highly specialised tools - Altium Designer, SolidWorks, IDA Pro, Altera Quartus, etc.
Upgrading the OS is not a trivial thing as would be on a phone or tablet. Also when upgrading the OS it would make sense to upgrade the HW as well, and that is a major investment. And Signal is just not important really to warrant that.
I would still use it on my phone though, but on the PC is just Viber unfortunately (whatsapp dropped as well).
If ChatGPT somehow ends up being the death of social media, i guess it is a win-win for the human race.