As a young piano student my piano teacher insisted I learn each hand individually before I put them together. I found this very frustrating because I never felt the work I did with my hands individually did anything to prepare me for playing with both hands simultaneously.
Learning to play one hand at a time, with the other completely uninvolved, does not prepare you to play both hands simultaneously. All that is accomplished is the illusion that the music has been learned. However the two-handed co-ordination needed to actually play doesn’t develop without two handed work.
Now some single hand study can be useful for working out specific technical problems or developing an understanding of a complex figuration. However the time should be limited and the opposite hand introduced immediately.