I agree with the 'why' question, Jar, but your second question has me worried. I think adaptation and imperfection are indeed the basic things that evolution is built on.
But they are not the only things. He (or she) tends to leave out the filtering part of the equation. That variable filter, one that changes with time, location, and circumstance is as important as the changes in the critters themselves.
It all depends on how you define 'design' of course, but if you call the relation that exists between form and function in many structures in living nature 'design', then I think John 10:10 has a point. We see that there is such a relation, so, in a way, we do "start with the knowledge that 'design' exists", just as we start with the knowledge that gravity exists when we try to theorize an explanation for it.
I think the key here is that first sentence. If some IDer can one day come up with a definition of design that is applicable to what is seen, they might be able to, at the least, begin a discussion and debate. But so far that has not happened.
Take for example the stone arches or rock bridges. They serve no function. A man made bridge is designed to cross an obstacle to allow folk, critters and things to get from one side to another. The rock bridge though serves no function. It just is.
But I don't agree that we start from a knowledge that design exists. There is a reason that we find natural bridges wonderous, and that is precisely because they are unusual, out of the ordinary.
To look at your example of gravity, it is something we know because we can observe it. It's a word we invented to explain what we see. It's not a preconception, but rather a result.
If IDers could first come up with some definition of what design is similar to the definition we use for gravity, "The thing that makes things fall down", then we could begin. The simplistic working definition of gravity above is still specific enough that all of us can then test it, and at the least, agree that is a working definition of the word, but not an explanation of the phenomina.
Right now we have no such definition for design. We could make one that is really broad, and say "Design is that thing that gives things form or makes them work or determines how they will react with other things." The problem is that such a definition is so broad as to be meaningless.
Aslan is not a Tame Lion