Because there are usually sections of the neighborhood that nobody owns, but still contain grass, bushes, trees, etc. so the HOA (home owners association) hires contractors to keep those places under maintenance.
Also, there's usually a set of rules that govern the neighborhodd, eg. yards have to be mowed before grass is ____ high, certain approved colors for housing, you can't rent a house out without living in it and owning it for ____ years (which is actually a great one). It's a safeguard that you won't have an eyesore house that's in disrepair with a unkempt yard that contains debris and shit. They can be annoying sometimes, but it does keep people from putting a black eye on the neighborhood.
I'm sure some are over the top, but with mine, they just like the illusion of power. By that, I mean you have to send a letter with a filled out document you print off online for housing changes like solar panels, new roof, massive landscaping changes like switching from grass to stone beds, etc. I've never been denied anything that I asked for, just finger wagged for not asking first when I neglected to do so.