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).
'08 E90 M3 - Alpine White - Sold
'13 E92 M3 - Silverstone II - Sold
'04 E46 M3 - Jet Black - BBS E88, KW Clubsports, Brembo BBK, SuperSprint Exhaust