If that were true, you'd think you see more steam exclusive games. The problem is I can't think of a single one I own that is steam exclusive. I am sure there's at least one out there.
Whether 31,000+ constitutes a lot of games depends on your personal definition. You said you'd think there were more, so I'm guessing you were thinking of a bigger number.
AFAIK developers generally don't announce when a game is exclusive to Steam, so that's probably as close to a definitive list as there is. And I don't know how accurate it is, either, as I haven't inspected it.
I assume one of the things that's missing from the list are games that used to be exclusive. E.g., Borderlands 2 was (on PC) exclusive to Steam for something like 7 or 8 years, but it's presumably not on the list since it eventually became a non-exclusive title.
(and I guess Valve first-party titles are a special case of course)
But regardless, the point was that steam features provided vendor lock in. I doubt that is the case for many of thees games. It's likely just that steam is the most popular marketplace.