You do understand that VTEC essentially changes cam profiles at higher rpms, right?
OK, with that in mind, a bike doesn't need to. As mentioned above, a bike doesn't need great mileage. A bike already makes much more power in the higher rpms.
Non VTEC engines can be made just as powerful as a VTEC engine. The main reason for VTEC is to retain low end economy, driveability, idle, and top end power in the same package. Great for a daily driver car.
Not many people ride their bikes at low RPM's to save on gas. If I had a streetbike, it would be at redline often.