And how would iOS/Messages know that recipient cannot handle HEIC? Eg. going to an Android user or recipient has iOS 10 and or not using Messages.
Not sure why the decision was made, but, if you email an HEIC, it converts to JPG. But not so with Messages. Yeah, odd that one but not the other.
ADD: does not help for Messages, but, some desktop apps will do the HEIC to JPG conversion. For example, I have the iMazing program for iOS device management and they have the ability to import photos and convert to JPG.
ADD2: unless stressed for space on the device, I'd just switch HEIC off, until needed. Makes life easier.