Because it provides a better experience, weather it's WhatsApp telegram, Signal threema or even discord or teams, they all provide a significant feature advantage over SMS. This starts with properly handling multimedia, not giving your number up to everyone else, proper groups, your messages living in the cloud for proper multi device functionality, your messages living unencrypted only on your device... There is plenty of real advantages with their associated side effects.
SMS as a standard is simply too old for modern expectations, this doesn't make modern expectations stupid just because the standard is not being kept up to date. I have not written a single SMS since 2013 or so, and my life is better for it. Also there is definitely open third party chat apps that provide an open standard that can just be installed as one of a few apps, the problem here is that potentially no one is using them.
The main advantage of a computer in your pocket against earlier phones is that you can fundamentally install any software you want, not just what the device manufacturer deems acceptable, so why would anyone not take advantage of this for messaging is beyond me.