It's encrypted, you get read receipts, you get notifications when people are typing, you can play games or use other apps within iMessage, and most importantly for me, it works via wifi.

All of those (and other things about iMessage) are big deals and aren't available on standard sms. Because of that, people with iPhones tend to socialize more with other people who have iPhones and less with those who don't. You don't have to download any app since it's the default. Unless you have a lot of international friends, you probably don't use or haven't heard of WeChat and WhatsApp.

Sure, you can say "fuck it" and just get an android, but much in the way I got invited to way less shit after deleting facebook, you end up being a bit of an afterthought.