It's not simply a MeanGirls experience of not being cool enough. Most Americans don't use or have 3rd party apps like WhatsApp, so most people will fall-back to SMS, which is objectively a much worse experience. I feel like the adult equivalent is getting group dinner with a friend with severe allergies or dietary restrictions. You care about your friend, and you want to invite them, but the effort to include them is high and sometimes you want to try a restaurant you know they can't eat at, so you skip the invite. I'm a vegetarian, and I know my friends skip me outright in the steakhouse dinners.
50% of Americans have an iPhone, and that is even higher for teenagers (almost 90%). That means >50% of people have this superior group functionality built-in (can't beat defaults). That means for teenagers, most of your friends will have iMessages, and most will be able to do effortless group chats, and its a statistical dice-roll to see if someone doesn't have an iPhone. You become "that guy" that causing disruption, and you'll 100% be ignored sporadically.
Again, the issue isn't "I don't wanna see green bubbles", the issue is "I don't want to bother with a third party app for this conversation". Since most people don't regularly use 3rd party messaging apps, there's a coordination issue to be solved picking the app and confirming everyone has it, OR falling back to SMS which is pretty messy. The alternative is to skip one friend and just fill them in later. Sometimes it's easier, it's not an elitist attempt to ostracize.
Apple also relies on the path of least resistance as well, if someone is having a poor experience in a group chat with their iPhone friends...it just becomes "easy" for them to choose an iPhone the next time they change phones.
Look at other companies, Microsoft porting Office etc to MacOS, Google services like maps gmail etc available on the iPhone. It's only Apple that walls their tech in so that it's only on iPhone - they don't care about profit lost to not expanding their reach because they reinforce their own platform.
Your current lived experiences may not be in sync with people in their teens or twenties. This is a well-known phenomenon called "green bubble bullying" that Apple has masterfully orchestrated to make people force other people to buy their phones.