You have people here cutting the extra webbing off their waist belts and other pack straps, so adding MOLLE webbing is a step in the wrong direction. Add-on pockets add an extra wall plus the attachments, zippers, Velcro, etc, so they are heavy for the capacity gained, and they aren't cheap.
IMHO, MOLLE is designed to attach heavy stuff--- far more than anything an UL hiker would normally add, so it is overkill at any rate. There is nothing in my pack that weighs as much as a couple grenades, loaded rifle magazines or other battle gear that is typically hung on MOLLE webbing. I rarely run, roll, crawl on the ground, grind my pack up against rough walls, or enter and exit vehicles or aircraft in a hurry with heavy loads. I don't need split-second access to my gear.
Outer pack attachments tend to catch on brush and other trail hazards. Stuff hung on the outside of a pack tends to sway and bounce, using up more energy and more opportunity for chafing and general clumsiness.
The pack I use has a top pocket and waist belt pockets already. That is more than enough, if not too much, for trail-side convenience and organization. Most of the add-on pockets offered for UL packs are designed to add a snack, compass, sunscreen, or other small light items.
So that is my take on why you don't see MOLLE webbing on UL packs.
If volume is a consistent problem, I would go with a slightly larger pack with good compression features so you can shrink it a bit when not fully loaded. It will always be lighter and less expensive to have a larger pack body than adding pockets.