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).
'04 E46 M3 - Jet Black - Sold
'08 E90 M3 - Alpine White - Sold
'13 E92 M3 - Silverstone II - Sold
'13 E92 M3 - Alpine White - Ohlins TTX, AP Racing BBK, Cobra Seats, Akrapovic GT4, BBS E88, Volk TE37, OE GTS - Aero, 1/2 Cage & Seat Delete