Articles in mainstream journals take a relatively large amount of time, money, and expertise to produce.
As do those articles in the first place. What is the incremental cost of making them fit for mainstream journals?
It is easier to simply collaborate with (and greatly influence) established mainstream academics who have already jumped through mainstream academia's many hoops (as Carl Shulman has been doing, for example).
Shulman is credited as co-author, IIRC. So wouldn't this still be SIAI publishing in mainstream journals?
So far, Shulman has usually been listed in papers' acknowledgements section. But if he's credited as co-author, then yes, that's publishing in mainstream journals in a sense.
Recently, I gave some reasons for SIAI to begin publishing in mainstream journals, and outlined how it could be done.
I've recently been made aware of some pretty good reasons for SIAI to not publish in mainstream journals, so here they are: