Consoles are vastly easier for most people, for a wide variety of reasons. Most people who have PCs don't build their own, don't know how to troubleshoot when things go wrong, and don't buy PCs with hardware specifically designed with gaming in mind (most notably in the graphics card department). With a console, there's never any mystery about whether a piece of software will run; if I have a PS4 and the game case says PS4 on it, I'm golden. With a PC, it can be a crapshoot considering the wide variety of different system configs possible even if you just stick to what's offered by major PC builders. And then there's choosing various graphics options, which, while awesome for allowing PC gamers to crank up eye candy, is seen as a major hassle by people who just want to jump immediately into a game and play it knowing it will work correctly with little manipulation on their part.
Also, over the past few decades, people have become accustomed to controlling games with a purpose-built controller, and while the modern console controllers work on PC straight out of the box, most people still associate PC gaming with "keyboard + mouse" controls that they assume they won't like or won't be able to adjust to (never mind the ignorance about how simple it is to get a PS4 / Xbox controller working with a PC; this goes hand-in-hand with the earlier point about ease of use).
Then there is the convenience of consoles in connecting to a TV so games can be played on a large screen from the couch. You can connect a PC to a TV / home theater (and mine currently is), but it's not as convenient as a console which is meant to always be connected in such a way and very few people want to spend more time troubleshooting their setup to get everything working in harmony. Also, if you are using a keyboard and mouse, doing so from the couch is... well, it's just awful.
Finally, consoles are also cheaper than a dedicated gaming PC. $400 can get you a nice enough computer for basic web browsing, but you aren't matching a PS4 as a gaming machine for that price.
So, with all those things considered, consoles represent a larger market for AAA game developers to target. They've got dedicated hardware that will run a specific level of eye candy, but you don't have to cater to different levels of system configuration so the people playing on grandma's laptop can play alongside people on quad Titans. The expansion of online connectivity has helped negate one of the major strengths PCs had over consoles in the past with regards to online multiplayer. I wouldn't say that consoles are the only reason AAA games exist, since if consoles didn't exist, developers would make AAA titles specifically for the PC (and some still do), but consoles certainly get the lion's share of the attention.