<<Guessing implies making something up without any supporting evidence. A hypothesis by definition is based on observable evidence.>>
Not at all. There are all kinds of guesses - - wild guesses, inspired guesses, shrewd guesses, informed guesses . . .
You assume all guesses are wild guesses, but they aren't. A good scientist won't make wild guesses, but he'll make a reasoned guess, eliminate some of the wilder ones, and then test his guess, er, his hypothesis.