How come Pagani Automobili S.p.A.can't have there own Engine...?

Why do they have Mercedes Benz Engine's??|||its not a MB actually its a AMG unit!|||shawn hit it on the head, plus also a little greed plays into it, and i mean you can't be MB power trains.|||Its cheaper to commission MB cuz the R%26amp;D on building their own engine would cost the company tens of millions of dollars.