Because if the nanites consumed the mass stored in Z-Space, the would not be able to
return the mass from Z-Space. They would once again need to borrow mass from Z-Space in order to sustain the host form, which would, in turn, result in a burn-out. The nanites would become confused, thinking that they were in a larger morph and glitch, leaving the host as a
nothlit... Of the host.
You're right, nanites are powered by your body. And thanks to Aluminator, I can explain/ret-con this a little better.
In a smaller morph, the nanites are stored in Z-Space, as we well know.
Now: We have previously mentioned Z-Space Tethering, yes?
While in Z-Space, the nanites are able to tether to the host form by locking onto a unique energy signature. Morph Energy. It's the trail of bread crumbs that they follow home.
After 2 hours in any given morph, the energy signature dissapates, and the nanites in Z-Space can't
find a host to return to.
Because the nanites are being stored in Z-Space, time also has a different rate of flow. 2 hours for the host may be instantaneous for the nanites, in very much the same way that several hundered thousand light years is a mere half an hour for an Andalite ship.
This would mean that the nanites don't particualrly run any risk of losing power, more that they risk losing the location of
when to return to. Morph Energy gives them a clear location, in our perception of time, of when/where to return to.
I hope this explains things a little better.