Because there is no reason to think that. Evolution happens quickly enough to be observed and studied (and of course the underlying mechanisms are being studied as well), and we are doing that for a very long time now (hundreds of years - ask botanists), and so far not a single reason to think of designers has surfaced.
The question is how much of the machinery in the linked page can you take away and still have a system that supports an evolutionary process. Once you're at the MVP for evolution, the next question is how did we get there from the initial conditions on Earth.
