Like others have said, it's loud, but it's also that it's a constant noise that can often tend to cut through whatever you're currently trying to focus on.

Add on the tendency people have to feather the throttle (do leaf blowers have throttles?), making the noise really inconsistent and unpredictable, and it makes it difficult to keep what little focus you have.

Sometimes, when there's like 3 different leaf blowers going at once, I can barely keep my train of thought if I can't drown it out by turning my music all the way up. :\