The technology was still developing. Early jets were fuel-hungry and unreliable, and the early prototypes that were flying from 1939 on weren't practical for combat aircraft. The He 178 was fast, for example, but not so much that it completely outclassed contemporary piston-engined fighters - the F4U, for example, would break 400mph a little over a year after the He 178 took flight. Gloster's jet prototype had more impressive performance when it took flight for the first time in 1941, but, like Heinkel's prototype, it suffered from pitifully short range and endurance that made it unworkable as a combat aircraft.

The potential of jets for combat aircraft was realized, however, but it took time for the technology to mature. Germany had more mature jet fighter designs in development since 1940 (the He 280) and the Me 262 had made its first flight in 1941. Allied efforts were only a little behind, with the American P-59 flying in October 1942 and the Meteor in early 1943. Development of a cutting-edge fighter took time, and the complexity of early jet powerplants was a major hurdle to development. The Jumo 004 that was the mainstay of German designs suffered from major issues with engine acceleration and had a pitifully short service life. Many of the more advanced German designs that never reached service were designed around the He S 011, which would never overcome its engineering hurdles before the end of the war.

Keep in mind that most of the mainstay fighters of the war had their origins in the pre-war years. The Bf 109 has its origins in a 1933 RLM requirement and it didn't enter Luftwaffe service until 1937. The Spitfire also dated back to similar development programs from the first half of the 1930s. American mainstays like the P-40 and P-47 also built on years of preceding designs and went through lengthy development before actually entering service. Pretty much anywhere you look, the aircraft that the war was being fought with were designs that were on the drawing boards long before war had come. Adding in the constraints of a wartime economy and difficulties of jet engine development, it's perhaps more surprising any jet fighters reached service at all.