> so I just use SMS
Get out of this 3rd world country *smug_face*
Honestly I do remember when I used the SMS lately - a once this year to contact a person whose contact was relayed to me through 3rd party. As soon as we confirmed what we are those who we are we moved on to Telegram. In the retrospect I should had just added his number to the address book and I would had seen what he is available on Telegram from the start, but I was in quite stressed state at that moment (hence the need to contact him, but that's the other story). But before that? Don't have an idea.
But overall people here just outright tell you on what platform they prefer to talk[0] from the start so you know it and your 'I need to ask people to install...' sounds... quite weird. Between all the people and different social media there is always at least WhatsApp or Telegram what everyone have.[2]
Another reason people have these apps is what they:
a) work; sure SMS do work, but sending a photo? Video? No way
b) priceless - sure, you pay for the traffic plan, but you pay for it anyway and it's extremely common now for providers to even give out a 'free' traffic for messengers. SMS still do cost money (at least for the crossprovider messages) and MMS... let's forget what MMS even existed. They are ugly and cost a ton. I'm not even sure young people, like younger than 25 even know what MMS is.
c) all messengers provide a way to conveniently send photos, voice messages (UGH!), video messages and place voice and video calls. All the things which are unavailable with SMS.
Honestly, after moving to 3G and further SMS doesn't even make sense - they are emulated on all levels and no longer a physical protocol. You can replace it with POEM (plain old email messaging) and it would be better on all occasions[1]
So yes, using SMS as anything is just another pointless American thing, along with price tags without tax and gaps under toilet stalls. Everyone else moved on because it's better. But because "it's the way" they "need it"(tm) and hence are quite defensive of anything what can even remotely change the status quo.
[0] and Instagram doesn't use the phone numbers, so adding to the phone book wouldn't help with autodiscover?
[1] hey, remember sms2email gateways? Email2sms?
[2] and amusingly enough I don't think most of people even knows what iMessage is, because even on iPhones they do use other messenger apps.