Because higher octane fuels keep detonation under control. Gasoline is ony stable in a certain compression range. If it exceeds its maximum safe pressure, it will pre-ignite causing knock/detonation. That's bad because it will blow your engine apart and melt pistons and valves.

Some fuels like Diesel and kerosene have much higher compression limits with low octane, but they rely solely on compression to ignite. Those engines don't have spark plugs, and achieve compression ratios higher than 50:1. The rods look like a cow's leg bones, and the bolts that hold them together are like trees in comparison to what's inside a Honda.

Some race motors are built 11.5:1 and are still streetable, but you won't get a B18 to run like that. I'm not sure it would stay together. What it would cost to make it do that in machining, you might as well bolt on a turbo because the stock compression on an LS motor is already turbo-friendly. Adding compressed air to the fuel mixture raises your engine's final compression ratio, in effect, doing the same thing, but adding more air and fuel, and in the long run, making more power.

You gotta run high octane fuel with a turbo, too... but you're not limited by your compression, you're limited by your tuning as to how you get away with running what gas.