[mikhailt]

Because you're using Apple's closed platform that only permits Apple-authorized clients that only exists on Apple devices. You're still bound by Apple's ToS for their services, nobody is entitled to unrestricted access to it. If Apple releases Messages for Android, that'd be them adding an approved client for their closed network.

Messages is not an open platform, one of its features is the use of open standard SMS for fallback to chat with your friends on any platforms but SMS itself never had any support for encryption.

Hopefully, next year, Apple will switch to RCS and the RCS industry will adopt encryption between all clients and platforms.

If you want to talk to your friends on an open chat platform that is open to any platform and any client, you can with services like Signal.