Subpar level design and shallow RPG mechanics are the number 1

But also presentation. Souls games have really memorable settings, soundtrack and overall atmosphere. As they were the first one, all the ones that comes next will be seen as derivatives

Oh YES, i didn't thin nof this, Bloodborne is tempting to Replay just for the world and feeling of it and it's not even nostalgia. They build worlds that feel exceptional. Hit the nail on the head. 

Hmm, asides from the really snowy area in ER, I can't think of any other that felt subpar. Maybe the haligtree slightly but DS3 has emaculate level design imo. 

Lake of Rot and Consecrated Snowfields are the only lazy-designed areas of the game, going as far as making half of the area with heavy rain so they did not need to finish all the modeling lol

Aside of those, they are all good. Even Mountaintops of the Giants is well-designed

Yeah, I figured that's what the fog was for and that whole area could have been cut if it wasn't for the haligtree being tied to it. I think it still has some redeeming qualities though, it's very tense, I was legit freaked out in that area a few times and could never tell when I'd stumble into one of the really hard revenants and the village with the invisible enemies, while frustrating is a pretty cool idea for a puzzle. 

The DLC map though, damn, that is so trash compared to the base game, I don't get why people loved it.  

They became popular, but camera and storytelling are bad (and they don't care because it's selling anyway).

Level design and atmosphere is Fromsoft games's strongest aspects and thats what makes them stand out - not just from games that try a similar formula, but across all genres.

Regardless, theres plenty of games that follow the style that I find pretty good and worth playing. Comparing them to the best there is and ever was seems unfair.

It's never easy to be as good as the original.

It doesn't have to be comparing them to each other, on their own merits they can hit that 10 or masterpiece level. For me Lies of P is a 6, Khazan a 7, Nioh 2 a 9. Yet ER, Sekiro and BB are such strong 10's. I have not found a souls clone that is a 10, Stellar Blade but that's a casual combat game and even in that, the story and VA is passable and the score is being held up very much in other areas it exceeds so well in. 

The breakdown in multiple areas in each game is probably better explained by others. But I think ultimately it stems from trying to make a soulslike clone rather than having passion to create their own game that just happens to be a soulslike. Hidetaka Miyazaki has the passion.