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.
What's funny is think of the "value charged but not added here". By all means I'm sure the parent contractor might do some light management to make sure delivery goes as expected but what do they take, a 15-30% cut compared to the subcontractor? All because they found + negotiated the original lead
Alternatively you could think of the value added as the general contractor is you loaning the subcontractor your reputation. Certainly that reputation should be on the line if the sub (and by extension the general) fails to deliver.
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.