system
Given the following post, respond as if you're commenting on Reddit:
human
[The Thing] Why does The Thing lose some of its shapeshifting ability after it assimilates someone?

When The Thing assimilates someone, it perfectly copies their every internal and external feature, right down to their memories and personality. However, after someone has been assimilated, they're limited to splitting open and creating crude appendages of previous assimilations. Why can't assimilated humans simply shapeshift into entirely different people/creatures that the Thing has assimilated before?
gpt
Transformations require time and energy. And the Thing generally needs a giant, toothy maw on the order of "*right fucking now*." And it doesn't seem to have a *perferred* form, just whatever is good enough for the job. It's more utilitarian that way, conserving energy between meals.