Because Mementos has less variety and the enemies in it tend to be recycled trash from previous dungeons?

The variety is the exact same. Randomly generated dungeon floors broken up into blocks as you progress. The aesthetic changes a little in both, but they're the exact same variety wise. Sure, enemies were recycled, But I never had any issues dodging enemies in Mementos.

Plus you know, Persona 3 has other dungeons too... they are just tiny and pointless.

Like what? The train full moon mission is literally on like a 5 minute timer. 2 fights and a boss fight in a literal line map do not qualify as a dungeon. I wouldn't count the hotel mission, seeing as it takes about 5 minutes to go from start to the boss fight.

Oh, and don't even get me started on that droning song that plays throughout the whole thing.

because this definitely didn't get annoying. And you spent a hell of a lot more time listening that than you did mementos music.

Also no floor effects which helped to keep Tartarus interesting.

Tartarus floor effects are essentially equivalent to Mementos weather effects.

You can pretty easily avoid enemies in Persona 3.

Not hard in Mementos. you can literally just drive right by them.