If you're talking about during the series, my recollection is that replicating the physical hardware was easy, but creating a complex neural net - the "consciousness" - and keeping it stable was incredibly difficult.
That's what made Lal so special. Data was able to keep a new neural net functional in that android much longer than anyone since Dr. Soong. So the technobabble answer is that yes they can create a body, and probably even transfer Data to it temporarily, but keeping that android stable enough for those memories to stay intact long term was unlikely to succeed.
Of course, IMO, this in-universe theory I think breaks down pretty easily. How is it that literally for decades, after intensive study of the androids, no cyberneticist anywhere is able to reproduce Soong's work? The man left no notes? Wrote no papers? If no one was able to reproduce it, doesn't it make the whole work somewhat suspect? They simply wanted Data to be unique and irreplaceable, and in the process twisted themselves into a bit of an illogical corner of an explanation. Then, partially undid that by creating first one, then two duplicates.
