Fighter jets use low bypass turbofans while large planes like tankers, airliners, etc... use high bypass turbofans. The Chinese engine industry has been focused on low bypass turbofans so they have progressed much faster than high bypass turbofans, the first of which are still in testing phase for the Y-20 and C-919.But j7 and etc are jets.
The answers above plus until Y20 and C919 lines get hot enough there’s just not a jet suitable available. Y20 could be converted to such but the bigger needs are transports and tankers besides overkill for the job. C919 is just crawling along as a program because it’s still brand new and heavily western based it would take time for the production like to shift to a more indigenous capacity. ARJ doesn’t seem suitable for the job due to the tail structure.