Can someone explain to me why people don't use the sidewalk, but instead walk on the road?

Well Several reasons why people walk on the side of the road.

I know from first hand experience, I have needed to

First: no sidewalk or shoulder of the road is available, and the grass is just too high (knee level), or very muddy like after a rainstorm and there on the way to an important business metting or job interview from the public transportation bus stop.

Second, there are obstacles in the sidewalk, or mud there

I could understand it after a snowstorm or something. Or even if it's downtown and the sidewalks are packed.

Cyclists are supposed to use the road. They are allowed to have their own space on a road, and you are to allow them the right of way. That you can't do anything about.

Perhaps scaring the crap out of a pedestrian on the road would help them remember the safest place for them is not on the road. Drive up behind em slowly and HONK!! Other than that, sorry man, just accept it, and keep your eyes open.

Many sidewalks are uneven and sometimes treacherous. At intersections or sometimes at driveways there are curbs to deal with. This makes sidewalks particularly irritating to bicyclists.

So it's just easier to walk in the street or road.

because most people are idiots and think they are the only one in the road.