Because nobody makes that size. AFS did in 03/04 style rims for a few years in an 18x10.5 but long out of production and they were pretty bad quality rims anyway.
Have you even looked for 315 wide tire sizes in an 18 and the price they command? And availability? I'm guessing not.
18x10 and a 295/35/18 is fine and is still kinda pricey sometimes. Brands that run big like BFG you can get away with a 275/40/18 which is what I did on my last car with 18x10s out back and the sidewall bulged out just a little bit past the lip perfectly. Because a 275 BFG is damn near as wide as a 315 Nitto which runs real narrow.