Partly because the watch doesn't rely on magic (with a limited exception in the case of Thud). Pterry's characters all rely more on moral integrity and personal strength than they do on magic, but only in the case of the watch is it truly absent. He truly mined the depths of Rincewind's possibilities and, in the end, usually relied on R's knowledge of magic and, sometimes, even his ability in magic. Susan relied a great deal on the semi-mystical nature of her quasi-heritage from Death. Granny is, of course, the most sternly moral person you'll ever meet, but even she relied on magic.
As I believe Pterry had a character state (Vimes, I think), there are no rules to magic. If you can make something up, magic makes it real. So if you can make up an explanation for why a character can do something, then they should be able to do it and now you have to explain why they didn't do it in earlier stories or why they didn't do it earlier in this story.
I mean, props to the man, he managed to come up with Granny's ultimate ability and make it both impressive and mostly useless. I speak, of course, of Borrowing. It came to a head in Lords and Ladies and then he gave it a totally unexpected twist in Carpe Jugulem. And he hasn't written Granny since.
I suspect that Pterry hates the gradual godification of characters that necessarily occurs, and also the calcification that occurs when a character settles too strongly into a role. Someone starts out as just being strong and fast and ends up invincible and infinitely fast and with heat ray/x-ray vision and is also his own great grandson from the future. Or he starts out as being an average guy with a strong sense of justice and ends up the inhumanly good personification of justice (*cough*carrotnobbycolon*cough*).
To that end, I think that he explores a character deliberately and carefully and sets it down before any of those problems (godification, calcification, magicification) can set in. He started out as a writer of some pretty hard sci fi, and I think that still informs his plotting propriety. He wants a logical structure, and magic is an escape from that. He also wants a consistent structure, and part of consistency, for him, is that his characters remain vivid and vibrant. Both of those words, incidentally, descend to us from Latin and relate to words meaning "alive".
