Because they are waaaaay to big.
You won't get any shit from me about ls1 in a skyrine Nick. Living it Gayton tho, that's another story.
A mate had a NA zed, spent many, many thousands doing a vh45 conversion (and 2 years). Sounded awesome but was still slow. So he pulled that out and went vgTT. Cheaper and faster.
Since parted it out and is now talking m3CSL or lotus