Because WhatsApp (and other messengers) offer a far superior messaging experience to SMS: higher fidelity media, on time delivery, delivery status, e2e, and many more. Better experience means that people actually use it.
As you say, for “extra security”. But not everyone arrived at the decision of “how much security” they need at the same moment in time. People come to and leave the WA platform, and it’s desirable for me to know where people are at now so I can use e2e in new places as opposed to only with the subset of my social graph that I manually copied and pasted in which had installed WA before me.
I don’t deny that you like your setup, but it sounds pretty painful and pretty unlikely to appeal to a broad swathe of people, which is essential when trying to combat mass surveillance.
(is mass surveillance why you jump through these hoops?)