Why does BitLocker not use RSA?
Asymmetric encryption is vastly inferior to symmetric encryption. That is, in all respects, except one -- being asymmetric. When that property is needed, there's no way around it, obviously.

Asymmetric encryption is much slower. It is much more susceptible to showing recognizable patterns of some kind given non-random input. You need much larger key sizes to provide an adequate level of protection, and the system is much more vulnerable in general with current and future technology (reasonably-sized quantum computers will basically mean instant death for RSA, but AES is pretty much "yeah, so what" in that respect).

That's the reason why asymmetric encryption is almost never used to encrypt bulk data. 

Nothing prevents you from encrypting a terabyte of data with RSA using 2048 bit chunks, much like you encrypt a terabyte with AES in 128 bit chunks. Only just, it doesn't make sense to do that because it is several thousand times slower, and at the same time is much more insecure.