>Because android sms is unencrypted

No. SMS are simply not encrypted regardless of whether you send them from an iOS device or an android device.

If you message on an android phone with someone else with an android phone it wont use standard SMS but instead use RCS and this is encrypted.

This is the same as using an iPhone with iMessage when texting another device supporting iMessage. The fact that iPhones and Androids don't have encrypted texts between them is due to Apple not supporting RCS fully.