I think aiya has it right. Currently, I believe all Homelink modules are made by one vendor, and the mere concept of a rolling-code capable RF transmitter built into a car may still be patented (but hopefully expiring soon if so), which is why there's no other vendors.
This is also probably why the additional expense to add it (now that it's not standard) is so high - not just the labor and some margin but also the fact that Homelink is probably screwing everybody with the pricing for the modules, when the actual hardware is unlikely to cost more than $5 in bulk.