Because its really immersion breaking, no one is going to issue WW1 vintage planes during 1943 even if things are not going well. Its just a waste of trained pilots.
That's not really true though. The United Kingdom was using leftover outdated aircraft to make up for shortfalls all over the Empire, as were various other nations, such as Russia.
For example, Gloster Gladiator biplanes were used to defend the island of Malta against the Italians in 1940, long after Hurricanes and Spitfires were introduced, as well as seeing plenty of action in Africa. The Fairy Swordfish, an outdated torpedo biplane, saw widespread action throughout the entire war, despite the invention of more modern aircraft like the Barracuda.
Even if they didn't use these again, they would keep them for training or emergencies, and eventually start giving them away to smaller nations like Finland.
The same is true of tanks. The Covenanter A13, for example, is barely heard of because it was outdated as soon as it was produced and never saw combat. Instead of scrapping them though, the British kept over a thousand of them in Britain so that they would have plenty of armour in the case of a German invasion.