alex567 hit the nail on the head.
I think of the ork Waaagh's as waves in the ocean. As long as there is wind to drive the momentum the wave continues onward, but when there is no wind the waves dissipate.
Imagine a sector completely overrun by orks. There's probably several warbosses fighting each other with massive armies, and all the orks do are fighting amongst themselves. Then suddenly, an incursion of (insert faction name here) occurs and the orks divert their attention towards the new enemy (because they are designed to band together when anything non-orkoid appears).
The biggest of the warbosses takes dominance, probably by killing the lesser contenders, and is made de-facto ruler of the combined ork forces. He is the one directing the massive Waagh against a (visible) target, but as soon as the immediate threat is over, the orks lose interest and through boredness, return to their petty squabbles as before.
The warboss can't be everywhere at once, and I don't think orks have any true loyalty, only respect for a bigger ork in their immediate vicinity.
Therefore I think Waagh's can only keep up the momentum by going planet to planet in hopes of finding new lifeforms to fight every time.
Also the orks have been designed to sustain themselves entirely.
The last orks are killed upon a random world.
The dying orks release spores that gestate and grow into a host of orkoid strains within months:
real orks (feral ork boys), laborers (gretchin) and food supplies (squigs)
The feral ork society will, if unprovoked, grow exponentially and suddenly more modern strains of orks will appear:
docs (doctors), wyrdboyz (sorcerers) and mekboys (mechanics / scientists)
The bigger the ork empire, the more advanced the ork society becomes, because only when at sufficient size will stompas and gargants appear, warmachines commonly seen during Waaagh's (Armageddon).
My guess is that orks can only stand to be crammed together up to a point. Then they move elsewhere or fight amongst themselves for territory. That's probably why orks are so succesful - they can grow from nothing and they will always instinctually expand.
That's where I see the Waaagh's 'gestalt psychic field' coming into play - I think it has a soothing effect on the ork psyche, enabling orks to unite whenever they have see and immediate common enemy, however it seems to be out of sight, out of mind for most orks.
A full-scale all-encompassing Waaagh would probably only be possible if you leave a breadcrump trail from ork empire to ork empire and then towards all lifeforms.
This is all speculation on my part, but I based it on what I've read in the old Gorkamorka books and Codex: Orks (3rd Ed.)
THE ORKS ALL SUFFER FROM ADD CAN'T YOU SEE IT???