The simple answer, of course, is for mutual intelligibility--because you have to pick one order, and there's no particular reason to pick one over another.
If you're asking about etymology, we can venture a guess as to what caused the split between English grammar and the German and French varieties. In Middle English and Early Modern English, you could form some questions using a construction similar to the French:
Lovest thou me?
was equivalent to:
Dost thou love me?
Note the -st ending on the verbs, which indicated the second person singular. English later lost this ending and the distinct second person singular pronoun.
At the same time, the "do" construction was used in the imperative as well as for questions:
Do you love him, madam; love him well.
You could also frame the imperative as:
Love thou; love and be loved in return.
Without the -st marker to separate "do thou love him" from "dost thou love him," or "love thou" from "lovest thou," there was an ambiguity. And speakers resolved that ambiguity, as of today, by reserving "do you" for questions and "love [you]" for the imperative.