Because there are some huge, genuinely great benefits to using it. No one really cares that much but, when the difference between the two experiences is as great as it currently is, it really affects the conversation.
For example, we don't really pressure friends to buy iPhones but we legitimately have a separate conversation group for just iOS users from the main SMS group chat so we can send images, video, or other media at full quality. Most of us are pretty tech savvy too so we're not really keen on using WhatsApp or Telegram or any of the other dirty data apps.
No, I mean "dirty data" as in all your data is collected and there's very little transparency regarding what those apps/companies are doing with your data. I haven't exhaustively tested all these apps but I have tested a few and know that Apple doesn't send any information or data from iMessage to their servers that isn't encrypted on-device. Unlike these other apps, Apple couldn't leak your data or usage from these apps because it's never collected in the first place.
For example, we don't really pressure friends to buy iPhones but we legitimately have a separate conversation group for just iOS users from the main SMS group chat so we can send images, video, or other media at full quality. Most of us are pretty tech savvy too so we're not really keen on using WhatsApp or Telegram or any of the other dirty data apps.