just kind of the nature of the genre for a lot of bands

I think I'm in agreement here, just a matter of phrasing it. It's very easy for a metal band to think they're stepping off genre in the albums they're making if they're a pioneer of some sort of subgenre (I think the most prominent example for me is Kamelot). So many of them end up making three or four mediocre albums that could have just been collapsed into one good one.

I also notice that some genres end up having really well defined 'tropes' that get established and then beat to death over a number of years. If you've ever listened to a band like Amaranthe, truly the Nickelback of power metal. They have like 5 albums and I can't tell which song is from where. Not to say that they're bad albums, or unlistenable, just kind of blurs together in a pleasant blob.

In comparison, Ghost really changed up their sound. They started off kind of like different Megadeth with a lot more theatrics (which is wild to think about), and now they're ABBA with distorted guitars... and more theatrics.

Pulling up something from punk(ish) land, Streetlight drops albums so rarely, and they're perfect shiny jewels every time. Not always totally fresh, but always putting a new twist on the last one.