Previous season form doesn't always give an indication of how they will perform. I have had players have great season and then slump, and the other way around (like a Scout that does well first season, sucks second, and picks up form next)
Also, if form showed many players would have to be sacked since they would be too hard to offload.. MC's & GK's often have crappy form but contribute to the Team.

I agree about morale, though. When going to a new team it would be nice if it went up, but people would complain if it went down! So I doubt that will change.