Because witches are compulsive clean freaks. Whenever you see one flying on a broom, you can be sure she's heading to her next cleaning job.
If brooms didn't exist, I expect you'd see modern witches riding carpet steamers and electric dishwashers (for really big witches who don't know any diet spells). ;-)
to smack the crap out of goblins...
I'd go for pimp my ride... lcd, navman, with sounds and fly first class...
They don't have good enough wings to fly.
They're eco friendly, ergonomic and get good gas mileage
Seriously it was because they used a broom for fornication and put a drug on the end of the broomstick. That's why they felt like they were flying, it was hallucinations.