I started working in a property management company couple months ago, and whenever I told people that I work in a property management company, people would assume I am selling houses.
First of all, most property management company don't sell houses, they rent it out. Most of the profits came from rentals, and selling the property is not a main focus for them.
Second of all, property management company take care of all the dirty work like maintaining the building, collecting rental, checking firealarms, making sure the elevator works fine. Whereas realtor doesn't care about the house after they selling it, its done business after the property is sold.
Third of all, property management doesn't require much sales skills and realtor requires a lot.
To make it simple, think of realtor as people who you would only deal with one time, and we are people who actually care about you(just kidding, we just wanna make sure you don't damage our property so we can rent it out right after you leave). 
Thanks very much for your article. It is very clear and nice.