Why does SIM toolkit not work with most custom ROMs? Most custom ROMs have problems with Stk.apk (the SIM toolkit part of Android) not functioning properly. The question is not the missing application (running Oxygen, I have the app but it is not working), but something systematic (dependency on RIL?).

I'd like to know why so, and what can be made about it?
Well, the SIM toolkit is initialized in 2 modes, dedicated mode and raw mode. SIM toolkit also depends on the modem you are using. If the modem does not support the dedicated mode, you won't be able to see the SIM toolkit in your UI as it sends raw data to the upper layer (URCs) in raw mode.