Presumably the difference between this and a "body shop" is that YC, having significant investment in the success of a business, would be less interested in "supplying bodies" than ensuring the business' success.
Whereas a outsourcing company would benefit by providing as many staff members as possible, if YC or the like were to do it, they'd likely want to zero out the "profit" on supplying employees: To keep it affordable for a startup, and to not break the incentive for YC: To push companies to success via whatever resources they can offer.
Particularly if YC's "cut" was solely in the form of share of the business, they'd have no benefit to over-pushing poor quality labor on their own companies: It'd just doom them.
Whereas a outsourcing company would benefit by providing as many staff members as possible, if YC or the like were to do it, they'd likely want to zero out the "profit" on supplying employees: To keep it affordable for a startup, and to not break the incentive for YC: To push companies to success via whatever resources they can offer.
Particularly if YC's "cut" was solely in the form of share of the business, they'd have no benefit to over-pushing poor quality labor on their own companies: It'd just doom them.