I wouldn’t go so far as to say it is the “real” engineering target, but it is a foundational capability that underpins the ability for humans to explore beyond the earth-moon system, and it is fraught with difficulty and uncertainty.
Fuel transfer and storage in orbit is problematic in many respects.
Very hard and foundational capability need not be correlated though. I think the more likely explanation that orbital refueling hasn't been done yet is not that's it's exceptionally hard, but that there hasn't been a need for it. Orbital refueling needs rapid reuse, and that has only been possible recently (with Falcon 9, and soon Starship).
Fuel transfer and storage in orbit is problematic in many respects.