A long time ago I was voted linesman of the year in Chicago. Did many pro matches when Mac and Lendl were playing the Volvo circuit and Martina and Graf were competing in the Virginia Slims. We would work the early rounds and based on performance be chosen for the finals. An average person in the stands does think they can call balls in or out, but for the most part they would be mistaken on close calls. You focus on the lines and not the ball. Training is required and linesmen still miss calls but for the most part see much better than most of the players do.
I never managed to chair any matches as umpiring is political to the point that out of town traveling umpires were given preference over locals. Most umpires in our area had the kind of job that they could play hooky from and they didn't really need the token pay of $20 per match. Think about the time spent to call lines on three or four matches a day and only make $80 and wait 90 days to get paid. Add to that, tournament directors placed umpires at the bottom of the list, often denying us simple courtesy considerations given to even drivers that shuffled players to and from the venue.
In the beginning it was fun and interesting, but as time went on the novelty pretty much wore off and I resigned from further events. Don't miss it a bit.