Why arn't engine blocks made of titanium?

As far as i can tell the only reason that would be stopping them is the cost of the material. However, in cases when they are building ZR1 Corvettes and other cars with a price tag of over 100k whats the drawback? I understand that titanium ways 1.77 times that of aluminum, but when it comes to strength and heat theres no better way to go. I think they could compensate for the heavy weight with the increase in horsepower.