It's partly the driver, partly the stack, at least on FreeBSD. My understanding is the newer high-speed wifi modes use multiple channels or pairs of channels or something like that, and the 80211 stack in FreeBSD was just not built with that in mind. So retrofitting it is some work. I think there are other things like that missing still, in addition to driver support.
I know of at least one open implementation of a wifi and BTLE modem made by a small university research team (though I can't find it online so it may not be published yet, won't say who it is unless I can find public details). It's definitely possible but it did take a team of grad students a little over a year to tapeout, and so it'd likely be very expensive
The RF side of WiFi is hell. The modem side is work, yes, but getting the air interface to work, and then dealing with all the interference management side is a nightmare.
I am not certain, but I think part of the 'blob' thing on WiFi internal implementations is to prevent someone breaking certification, particularly for the interference mitigation for things like DFS.
Intel and Broadcomm hand you a binary firmware blob and tell you to go screw yourself for the most part.
Atheros is one of the few helpful ones. The wifi support that we have now is the result of painstaking hard work.
And to be honest, I use the wifi on OpenBSD fine without 802.11ac support