Because they are scared of leaking their weakness.
You are right, that's called insecurity. T50 or Pak-Fa is not at par with real 5th Generation birds like F22 or J20, the main birds of superpower air force in the next 30 years.
But I do agree they can compete with F35, the marketable new generation jet for world countries. India, Japan, Australia and many other countries could be potential buyers.
(I don't know if Russia will still have the chance to come up with real competitor of F22 and J20 in the future, if their techs and cash reserve allow, but T50 definitely won't be the one that can challenge Americans and Chinese in the air.)