iMessage is a typical American thing which, we Europeans, have a really hard time comprehending what is the obsession with it.

To help you comprehend - the big difference is SMS has been free for a long long time in the USA. No other text messaging service has ever been able to get off the ground because why on earth would anyone sign up for Viber / WatsApp / Messenger / Signal / Telegram / Threema / etc, when you could just use SMS which works fine and works for everyone?

Then iMessage came along, and you could keep using “SMS”, only now it’s more reliable, has high resolution photos, delivery confirmation, etc. That was a real improvement over SMS, with no cost at all other than having to stay on the iPhone platform, which you were already on, and who’s going to switch? You’ve got all these apps you found/like and who knows which ones work on Android?

Also, it’s not just the USA. iMessage is big in other markets too. Also ones where SMS has historically been free. The cost of having to pay to send SMS between London/Paris is a pain we never really experienced here, so there was no motivation to try WhatsApp/etc.