US is doing it because they lack production capability to quickly replace these old aircraft. PLAAF has shown that it is not willing to upgrade J-10As and instead just retiring them to training units once their time is up.The americans are upgrading old F-15, F-16 and F-18 with AESA, new electronics and weapons, China not doing same with the newer J-11B and J-10A would be criminal negligence to say the least. Some months back we were going back and forth about J-15 not having PL-15 until PLANAF indulged us. PL-15 is even available on JF-17 so why not on J-11BG? A J-11BG with J-16 AESA and electronics, or perhaps more likely derived from those on J-11D, would actually be a bit better air to air kinematically because it's probably a bit lighter and less draggy.
J-11BG from most sources is claimed to be a minor upgrade (technically speaking there isn't even definitive proof to say it has AESA), some even claim it might just be a antenna upgrade to replace the worn-out slotted planar array. Upgrading J-11Bs to J-16 or even J-11D standards will be impossible without major overhaul of the entire electronics system which is likely going to be very expensive.