I had a doctor once tell me that because he went to school for as long as he did he could write any way he chose. I told him that being that he did indeed go to school for that long he should have also picked up a bit of humility.
1. Because they have to sign things a zillion times per day.
2. To make it more difficult to fake.
FYI- they dont WANT to improve their handwriting. Its sort of deliberate.