Why do they always wet the sidewalks and streets for film shoots?

You've got the right answer. It makes the streets look better on film. There is more reflection of light off of wet pavement, more shine or gloss.

In fact, I started to use the same technique at home by first spraying my garden, stone pathways, etc. with water prior to snapping a photo. It makes a difference in the outcome. More sparkle!

I believe it's more for cleanliness. They clean the streets that they want to use so that they don't look so bad for the camera.

It also keeps any dirt from flying around.

i dont think is water in the sidewalk, its lighting