Consistency which is only possible in an environment of collusion.
You don’t have any other options for most of their respective platform and there isn’t enough incentive to compete on the pricing.
Also even when there is competition there are rules that prevent you from lowering pricing in one store vs the other (there are some exceptions) so if you end up say selling a game on the PC and you set the retail price as $60 and you launch it on 3 different digital content distribution platforms while you might make more money on one platform than the other there is no difference for the end consumer so there isn’t an actual competitive incentive to reduce your cut as a digital distributor unless you think you can get an exclusivity deal.
If the first store did 20%, it's likely all the stores would have followed suit on that price point.
30% is likely the most they can take that people will tolerate.
The bigger question is whether their cut should be 30% in the first place.
30% was chosen by the first store of this type and it's just never been questioned by platforms. I just read that Valve recently changed their agreement to be a sliding scale based on sales (30% to start, falling to 20% if you hit certain numbers).
But as far as I know, Microsoft, Sony, Apple, and Google all do a 30/70 split on digital sales.