They did have them on other types - the Hurricane had 44 gallon drop tanks, the Spitfire had several types of tank it could carry, and drop tanks were available for use on the P-47 by the summer of 1943.
Also remember that it wasn't until the summer of 1943 that the USAAF was fully convinced that escort all the way was required; faith in the combat box with a formation protected by dozens of heavy machine guns remained fairly high to begin with. I forget when the first USAAF Mustangs were fitted with drop tanks