There's only so far your high tech nano-particle swarm can spread before resources become a serious limiting factor. If you're limiting yourself just to earth, certain resources that a swarm might need to reproduce will be scarce/energy intensive to extract and utilize (mining rare earths, fabricating highly advanced semiconductors, etc.). It's extraordinarily easy, however, to produce billions of cells nearly anywhere on earth because the cells that we have are especially adapted for ease of reproduction in these exact conditions.
There's only so far your high tech nano-particle swarm can spread before resources become a serious limiting factor. If you're limiting yourself just to earth, certain resources that a swarm might need to reproduce will be scarce/energy intensive to extract and utilize (mining rare earths, fabricating highly advanced semiconductors, etc.). It's extraordinarily easy, however, to produce billions of cells nearly anywhere on earth because the cells that we have are especially adapted for ease of reproduction in these exact conditions.