I've always thought it's a combination of the genre being niche and being founded on a set of lyrical themes that are restrictive - and maybe particularly challenging -, and also, the fact that a lot of these bands aren't from English-speaking countries. The bands from anglo countries like Kamelot tend to write acceptable lyrics within the power metal context (Manowar lyrics are hilarious at times and arguably well-written, but they are not power metal IMO). There are some noteable exceptions to non-anglo bands with polished lyrics, like perhaps Nightwish or Blind Guardian, but a common occurrence otherwise is to find shitty English, which at least for me is an instant turnoff.
In one of the raw scripts I received from the German side, I remember staring at "medicament" for a long time, before looking it up.
So yes, I know the feeling. :)
One thing I learned ~15 years ago was that Nightwish commissioned lyrics from lyricists (one such commissioned person told me himself I believe a year or two after Dark Passion Play was released - they never used what he wrote for them though) - maybe that helps give the polished impression for some.
(Of course, there's only so far you can push this—some metal bands are much closer to pop or jazz, and some classical music is quite distinctively its own thing.)
Yep, there's a whole bunch of bands in the symphonic deathcore genre. Shadow of Intent, The Archaic Epidemic, Lorna Shore and Immortal Disfigurement are a few that come to mind.
I often hear the joke that Beethoven wrote the best guitar solos. Moonlight Sonata's 3rd movement on guitar is a fun listen .
Have you heard neo-classical metal? It's shredding at its finest. Yngwie Malmsteen is the OG, but there are many others I recommend like Hizaki and Syu. Guitarists in this subgenre often release solo albums (no vocals) so you won't find yourself distracted by lyrics.