I do not know ANY of the providers, stakes' worst decision was to let the game building go and just hire from others, it never really made sense to me. I can't be bothered by it because it is just optional, they still have their own games that I play on and that is good enough for me, the only downside is that they are not adding any new games themselves, they are just dealing with others and add their games which is really zero to me, it means they didn't add any games.
The fact is that provably fair is not something on those games and only applies here on the original games is good enough reason for me to now gamble there. Why would I want to gamble on a third party provider? I am 100% in support of third party providers being here, because I am not forced to play them, but I want more games from stake themselves as well that are all provably fair.
Just one word that can explain everything: competitiveness.
To increase the number of users they need to attract them in some way. Many other casinos already have third-party games. The more providers of games, the more games, the more different the promos, etc.