Then I can drink when I want, not squirt it up my nose, and have no cups to worry about. Also, no issues if the race runs out of water (not unheard of on unexpectedly warm days), and no slowing down to grab a cup or risking bumping other runners who are also trying to grab cups.
Few people ever ask me to sponsor them/donate. Those requests that I do get are usually emails, or texts, telling me about their Justgiving page, which is easy enough to ignore
When I ran the London Marathon a few years ago, I had my club place, so no need to raise any money.
So many people offered, unsolicited, to sponsor me (assuming that I must be raising money for charity) that in the end I decided to accept if it was offered, but I never asked anyone to sponsor me, or make a donation.