Is it some sort of patent that Intel owns for m-itx mobos?
No. It's because the AM3+ package uses 2 chipset and other stuff that takes space on the board. Even their Micro-ATX boards are not very good for that same reason.
There are a few of them around, but I think they have all been discontinued. And there are three am3 itx boards on pcpartpicker, here
,here, and here
OP meant AM3+, not AM3.
Big chip, big bus, big heat, small size... Just not a good combo really