Why won’t the in- game ATC assign me climb/descent altitudes?

So i’m relatively new to msfs, and i recently came to know about IFR flights and simbrief, i currently use the Fenix A320 and FBw A32NX aircrafts and also came to know about simbrief. So here’s how it goes, i generate a simbrief flight plan, start my flight at the departure airport’s gate, load the flight plan into the Fenix’s EFB, do all the init data req stuff and set up the flight plan in the MCDU. What’s weird is that the in game ATC would just presume i’m flying VFR and say something like cleared through Charlie airspace, and not assign me the correct climb/descent altitudes as mentioned in the flight plan.

Any help at all would be appreciated. Regards