The thing you are missing is that many of us have friends all over the world. And international SMS quickly gets expensive. (Also, most people are not on signal / your-favorite-messaging-app) ...

Group chat doesn't exist in sms.

Also it is quite old fashioned and painfully limited wrt/attachments.

And expensive for thise with families/friends abroad.

People make group chats in FB messenger. If you don't have messenger, you don't get to be in the group chat.

To give my personal answer for it, there's no other cross-platform messaging service that everyone uses.

There are other chat services sure, but the Facebook has by far the most number of people I know on it. Also, not everyone I know exchange numbers. There's no usernames, etc on Facebook so its easy to find people too.

Weird, I know, but guess generations are changing. Apparently now Snapchat has become the main form of 'communication' for many millennial with their friends. So looks like I'm falling behind.

Because SMS only supports text, but not any form of multimedia. Granted you can use MMS but that's really expensive and only does photo and video. Also, SMS has no encryption or sender authentication, not even any way to prevent MITM sniffing or spoofing (which is why online banking SMS verification is unsafe as hell).

My friends use messenger and getting them to change is hard, not sure why they migrated to it in the first place...

SMS isn't cross-platform and costs money.