Because, as I said above, I see no compelling reason to assume design in the first place if I don't know exactly how something "came about", especially if it doesn't resemble any artifacts made by designers that are known to us (e.g. humans) and it looks as if it is the product of some long and messy natural process such as evolution.
Complexity is not necessarily the result of intelligent design. Usually it is the other way around since an intelligent agent should design a system as simple as possible and not unnecessarily complex.
Of course you can always postulate a designer who, for mysterious reasons, inscrutable purposes and with unknown methods, made all living things to look as if they evolved from a common ancestor but such an hypothesis doesn't survive Occam's Razor.
If you accept such an hypothesis you can as well accept the one where the Designer made everything Last Thursday and be done with it.
Occam's Razor will not lead you to believe that a rose and a robin came from a common ancestor.