Why? The engineers know how to build an engine block sufficient to the needs of the application. The block must support the crankshaft which is taking the firing impulses of the pistons and passing to transmission. But the block is not a wearing part . . . . that's the bearings, cams, piston rings, valves, cylinder walls etc. Making the block heavier or of more expensive materials is just waste. (And - I don't know this for sure - isn't titanium strong - but more brittle than iron, steel, alum, etc?)
"Back in the day" - most engine blocks were iron. . . . . but Hudson made their blocks of high-chrome content alloy. SInce it was a flathead engine, that meant that the valve seats resisted wear better. And the piston rings wore faster than the cylinder walls. And where did spending more money than necessary take Hudson? Or Packard, or Peerless, or Pierce etc etc? !