Why did most WWII Allied piston engines not use fuel injection? One would think that given the huge push for ever more performance and reliability on piston engines during the heat of WWII would push the designs to their absolute limits, however, most American engines seem to be comfortable with the old carburetor technology which is prone to icing, hiccups and perform poorly under high Gs. Reading the literature of how Pratt &amp; Whitney optimized the Double Wasp to an extreme (to the extent that the crankshaft is taking so much force that it needs a pair of torsional dampers even after forging) is quite jarring when one comes to realize it still uses the good old carbs to suck in fuel, when at the other side fuel injection was adopted eagerly.
Those engines didn't use carburetors as in gravity operated float carburetors.  They used "pressure carburetors".  Pressure carbs are mechanical fuel injection systems that inject the fuel from a single point at the start of the manifold instead of injecting fuel at the intake port via a distribution system.  Not quite as theoretically efficient as point fuel injection, but it has all the other advantages including automatic mixture control and no G sensitivity, as well as being less sensitive to damage by not having small high pressure fuel lines running to each cylinder.

On cars in recent years you had a similar system called "throttle body fuel injection", more or less an electronically controlled pressure carb.