So, why can't high level magic items roll % health regeneration?

Is it because it's too powerful?

Or is it too much of a pain to code in % health regeneration along with flat health regeneration?

I mean, think about it. Life on hit can roll, but so can % life leech. Flat accuracy can roll, but so can % accuracy.

Why not % health regeneration?

Is there some reason against it that I'm missing?

Because it's strong as fuck.

Since they can make them decimals, and they can restrict such mods to just one or two item types, I think it is pretty possible to have those in the future.