That didn't seem to be the case with Season 1 or Season 2.
Yes, it was the case, it fell into the "or they don't exist at all" category.
It's not a special feature issue. It's a how much can CBS suck the money out of the consumer because this is an expensive project issue.
It's an issue of budget
. CBS is not going to greenlight additional spend on interviews/documentaries above and beyond the norm if it's just going to be part of the season set. It raises the cost for them while resulting in no additional sales. That would be a dumb
This is the same reason certain episodes were split into two - for budgetary reasons. They get more money for a two-parter than a solo episode, and have the ability to put out a bigger and better product.