It is not hard. The app store usually take 30%. That is A LOT and that number is growing when you sell a lot of copies.
But you save money, because you do not have to print Discs (normally more then you expect to sell in case the game is successful), you do not need huge logistic to bring the discs to the customer... In the end digital seems to be a good deal for most devs...
If you sell a decent number of games dev costs should be lower if you do not know if the game is a smashing success and it should not sink the company if case of a fail.
Note that with Steam if you sell beyond certain thresholds your fees get lower. But also consider that you save in bandwidth, save on a lot of network, controller, etc dev work with Steam works, save on patching infrastructure, and much more.
Could Steam feed be lower? Possibly so but then we would not have the output around Linux, SteamVR, BPM, controller support and other things Valve is spending money on. So yeah, fees are subsiding feature development.
Now let's compare to say Ubi, EA, Take 2 or Epic store. As a customer I know which approach I like better.
Also, as a developer you can sell Steam keys on a multitude of sites such as GMG, Fanatical etc... for much lower cut.