Steem used to be mined long before I joined. There are witness "producer" rewards, but I am not sure how they function or. see for example https://steemd.com/@yabapmatt
It also appears that witnesses not in the top 20 can still get producer rewards. see https://steemd.com/@klye
I've never set up a witness node. I hear they are hungry for RAM-and ideally you need hardware that is probably better than what is in your typical budget home pc [not that I shopped in years]. But I would suppose that anyone who has the money for such a system, a good enough internet speed, and knows how to use or is able to learn how to use unix/linux could run a witness node. There are cost associated with running a witness node. Ignoring any labor costs they include: the financing of the machine; the electricity to run it or for cooling, and; maybe a higher monthly bill for internet. This may not be profitable nor possible for everyone.
People get to vote for witnesses, and the rank is based upon the sp of those votes. The old saying, "building a better mouse trap isn't enough" is true. Steem is a better platform than tron, but tron won in the marketing department. To be a top 20 witness doesn't mean going out and making a custom made system by itself, but marketing why you should be voted on as a witness....particularly to the major stake holders.