Leech can end up with a high value because it is relatively "cheap" point-for-point compared to other stats.
We generally let our tank simulator figure out whether a stat is worthwhile or not. It's easy to think that leech is kind of useless... you get all these tiny heals that seem meaningless. But... getting a small amount of stamina also feels kind of meaningless too. Am I really going to notice if I have 6.1m health vs. 6.2m health? But stamina is clearly one of the best stats for staying alive.
Leech can be thought of as effectively increasing your health: you get hit, then leech heals a little bit of it up for you before you get hit again. All self-healing can be thought of in a similar perspective: it increases your health at the time you take a hit by healing you back up after the previous hit. Stamina makes your total health pool bigger, so you don't go as low when you get hit, and you can just take bigger hits in the first place.
If leech was largely useless and just causing more overheal, the tank simulator should ignore it and think it is bad. But that never seems to be the case... getting some leech definitely makes a difference, and just like with other stats, you don't need a lot of it all at once for "any" of it to be good -- every little bit has value.