The problem with the concept of "it should just automatically" convert the video for MMS is...exactly how do you plan to do that? HD vids are very large, and it would take a ton of compression, not to mention the burden on the RAM and processor of the phone each time it had to do this. There was a time when phone makers did build similar tech into their devices, but we're talking about days prior to real HD video.
If the iPhone can actually do this, I'd be shocked. Not saying it can't, because I've never owned one, but I highly doubt it. And I'm talking about actual MMS, not iMessage to other iPhones; I believe I read somewhere once that iMessage sets limits at 100MB -- obviously this is much higher than standard MMS, but still vids in high def would hit that mark pretty quickly.
The cloud services folks have mentioned are better means for sharing the vids, which makes the option of trying to support compression of HD videos for MMS just seem like a waste of R&D (though that's just my opinion).
EDIT: part of my point was that iMessage is a great, often over-looked service that Apple offers, I do not deny that. But it is also iMessage that allows folks to send larger video files via the messaging app on the iPhone. True MMS only supports file sizes so small that compressing HD video to fit would be nearly impossible, and unfortunately, there's no cross-platform solution with Android to use with a service like iMessage... In other words, the OP may have a legit gripe, but may have missed what the root cause of the issue is.