Knuth is from a time when professors had secretaries who would type their handwritten manuscripts (maybe he still has one). That's why he thinks it is OK to handwrite all his papers and even code. Normal people (us) may even think that handwriting is easier than typing, but very few of us are willing to spend the time it takes to both handwrite and later type the same document. So, in my opinion, the reason for the choice is pragmatic, given the way we need to prepare software.