We just did it to be sure they didn't crash on someone's head on the sidewalk and to be sure they were insulated well from rain or wind, etc. 

This is a small one, but in my first rental it was only spoken that the tenant would shovel the snow and salt the sidewalk to melt the ice.

We also had units on the 2nd floor right above a busy sidewalk and didn't want them falling down on people's heads. 

One roof, one sidewalk or parking area to maintain, one tax bill, all maintenance in one location, etc. 

They are a combination of roof pitch, dormer count and sizes , covered porch square footage, attached garage, 16 inch eaves, 2x6 construction, driveway with sidewalk leading to front door and more.

I'm watching a sidewalk being built in my neighborhood and in order to continue it all the way down the street the city would have to go more than 10 feet into some of these properties because of trees.

First, unless you have a good bit of experience and have done this before, I doubt you'd hit $60/sf if you had to put in utilities, separate meters, sidewalks and other common things you'll find with multi-family development. 

By 10 pm I had 300 boxes to check on the sidewalk.

I dug 6 inches below the sidewalk grade, trucked out about 30 1-yard trucks of soil, packed in 4 inches of concrete base and 2 to 3 inches of gold-colored rocks.

What if the sewer lines go, and the sidewalk needs to be torn up with a back hoe?