Because they have too much business to actually deliver, and would get a cut if you did some of the work for them; it's called subcontracting, and it's very common.
Though from what I hear this isn’t necessarily the case in construction / home improvement.
But in any case, if you can’t close the deal yourself, 70-85% of something sounds better than 100% of nothing.
My reputation is worth a fair amount to me, but I’ve never been a general or sub contractor though so my opinion is probably pretty naïve.