Because if it doubles itself every time it adds up fast.
2 4 8 16 32 64 128 256 512 1024
Even as big as the universe is, it starts getting getting out of hand pretty fast. You have a million of those things in 20 splits.
And we don't know what resources it consumes to do it. Something that big has to get fuel from *somewhere*.
Of course if it does that once every million years that's not too bad but if it does annually? Or monthly? However big space is that's going to become a problem for *someone*, *someplace* eventually. Especially if gets more complex.