They don't, but they sell physical copies of PC games as well.
So all the same costs associated with consoles games applies to PC games. Apart from the licence fee paid to console platform holders. This cost for PC games is in the form of the fees paid to Steam in the vast majority of PC game releases. Definitely in the case of Fallout 4. So the poster who claimed the PC version of this game should be cheaper than the console version, because there's no licence fee, was mistaken to think that similar costs didn't apply to the PC version. Hence its similar price to the console version.
Doesn't matter though, they'll all take a similar cut regardless of the outlet, with Microsoft/Sony taking another cut, but only on consoles. There is a very good reason that games are usually a flat £10 less on PC compared to consoles, it's just a fact pal.