That's true, but a little disingenuous. I bought my PS4 (a bundle with Bloodborne) for £250. If I were to build a gaming PC, the OS alone would be almost half of that £250, with £150 left to buy the hardware required to play newer games on par with that PS4. That's also not including peripherals (mouse, keyboard, headset, potentially a controller for keyboard-hostile games like DaS), which are all included in the £250 for PS4 (headset, controller). I don't include the monitor, because anything with a HDMI port can count as a monitor.
There's no way you're going to get an equivalent PC build for the price of a console, even factoring out the regular upgrades required to keep up with system specs. Not only that, but you need a degree of expertise to know what brands to buy, which games have video card-specific issues, which sites to look for deals on, know how to fix bugs, etc.
With consoles, you just need to stroll into pretty much any major shop and exchange money for a fully working, fully optimised, fully operable video game platform that you can assemble in less than 5 minutes.
In regards to sales, there's the preowned market, which sells Battlefront for £5 (it's £11 for a new physical copy on Amazon at the moment, compared to £20 for the physical PC version, and £16 for 75% off on Origin) and Doom for £29, which is equal to the 40% sale currently going on for Steam (its regular price is £40). So, the sales aren't even that beneficial, either, or at least not for the AAA titles.