Some have threatened to do it...
The biggest issues are that of licenses (GPL vs. closed source commercial), and the fact the OBS Studio uses QT.
In looking around at GPL-compliant implementations of the VST format all of them use GTK as their display engine. So we would have to port/re-write their workings from GTK to QT just to get the interface working... seems like only 1 person has tried to do it and make any headway... didn't last for very long.
Then there is the issue of where did these GPL-compliant implementations come from? If they aren't clean, then OBS would be liable for copyright infrigement if they were included/copied into OBS.
So I'm not saying it's impossible (same thing for ASIO support), but it's a bit tricky and given how much work is to be done in other areas it's not surprising that (at least at this time) ASIO and VST are not on the planned features list.