Cars have titles that allow the leasing company to maintain contractual ownership of the car in order to secure the lease cleanly, efficiently, and cheaply. Aftermarket parts do not have titles. The cost to document an aftermarket parts lease just wouldn't be practical (each transaction would be custom).
'13 M3 - Silverstone II - Sold
'04 M3 - Jet Black - BBS E88, KW Clubsports