Why do mint-looking LPs sometimes play poorly?

Groove damage, they've been played with a worn stylus
Poor pressing
Not cleaned properly, use disc doctor

That happens to me sometimes too. It's a bummer. I just paid a premium for a great looking version of Led Zep IV and then steam cleaned/Walker Audio Enzyme on my VPI machine and it's really noisey.

Bummer

Unlikely with Japanese, but dirt pressed into record at plant... brand new, still has surface noise.. also, brand new Japanese pressings can sometimes have a harshness esp. near inner groove areas, ? don't know why...
Second on groove damage, sometimes mint LP's have hacked up grooves but you can't see it... been there know all about it.
Conversely, sometimes dirty record bought used, once cleaned well, plays great like it was never opened!

Probably the original owner wasn't an audiophile and played it with wrong settings, too much VTF etc.
or
it was cleaned with RCM fluid which changed the structure of the vinyl. Vinyl from Japan is generally the very best when new.

records which are subject to poor storage conditions for decades can look spectacular. even some that are sealed.