Games aren't "next gen" enough for them.
I don't have a problem with reviewers being a little harsher this gen, problem is that that wasn't communicated with anyone so a lot of people still operate under the assumption that everything has the same standard as 5 years ago, metacritic not changing their coloring system doesn't help with that either.
Imo you can add on average about 5 points to an aggregated score nowadays and you'll get about what the game would've gotten a couple years ago. Exception would be Nintendo games, they are still reviewed the same way as in gen 7, kinda appropriate to think that Nintendo is a couple years behind in that regard as well.