There would be less reason for people to buy iphones?

Apple products are an ecosystem. It's not just the physical devices they're selling. It makes sense from a business perspective to keep iMessage on iOS only, because it keeps people in the ecosystem.

people have been asking for imessage on android for years now and i really don’t know what the issue is.

Apple wont until somebody makes them.

The default text messaging app on ios is also stealthily a fully featured chat app that means that less tech savvy iOS users just think their texting is better.

The blue bubble green bubble issue isnt because ios users hate android users and the color green as much as it's bad, it's because big group MMS chains are TERRIBLE. I remember having a friendgroup in the 2010s that had a big mms group chat before we switched to groupme and then eventually telegram(we would have done google messages if google hadnt abandoned it began its not infamous chat sabotage). It was bad. Low quality media, the protocol updated in batches so sending and receiving wasnt as smooth as a chat app, and also you technically could just leave. We eventually switched to a platorm agnostic solution because we had diverse devices.

Now in the US iphone ownership is like 60 percent. I imagine the ratio is even worse for younger users. So for these users they use default messenger with most people and get feature rich multimedia chat app without even looking for an alternative provider. In some cases I imagine there are users young enough to not even realize what actual text messaging is or if they do there are chunks that think apple just texts better. It gives a perception of quality.

Then they invite Johnny Android and suddenly the chat degrades back to 2005 flip phone quality SMS/MMS. Now this is entirely apple's fault and the inability to chat with android users is technically their problem. Or it would be if iphones werent so popular. From their perspective they were having a good time using their default cell phone messanger and the android user broke it. Likewise unless Johnny android is particularly popular or charismatic it's likely they wont switch to telegram/signal/whatsap/line/etc. They might just leave Johnny Android out of group chats because his presence isnt just green it makes chatting clunkier and worse.

Johnny android might feel left out even if he isnt being outright bullied and may want to switch to an apple device. Likewise the iphone users may not want to leave the ecosystem and then not be able to easily communicate with their friends.

SMS fallback I believe started more or less as a way to deal with inconsistent data in the early smartphone days but the unintended consequence of killing off the appeal of third party chat apps on their platform and creating a weird social bubble was a happy accident and apple is never going to give that up.

Kids beg their parents to buy them iPhones and parents do it so their kids dont get left out or bullied. Even adults are pressured into this and it can be a factor when online dating.