Tanker versions of the TU-16 Badger and the M-4 Bison were flying in the early 1960’s. Il-76 and 78 in the 1970’s. Not even close to “never bothered” or “70 years behind”.

As Graham points out The MiG-21 was originally built as a point based interceptor. Point based interceptors were not designed for long range/endurance. Maximum performance, especially climb, were emphasized. This results in a small, light interceptor. MiG-23 followed this philosophy, with maximum speed being a key parameter.