Orikoru
Tour Winner
You say that but we've all had bad waiters and waitresses haven't we? So there obviously is a degree of competency involved. Plus I think of it as the bill being for the food and the tip is extra for the serving staff specifically.Why wouldn't you expect a caddy to be any different to a barber or a waiter?
I don't often use, or tip Barbers, but waiters have literally no skill level compared to a caddy. You could pretty much teach any able bodied person to be acceptable at this within a few hours.
A caddy has to know the course, has to reasonably quickly work out your game, has to know the lines or be able to read putts, has to give you yardages subject to the conditions, elevation and has to be fun and engaging for 5 hours on a golf course, possibly suffering the company of 4 absolute toss pots.
As I said, if people didn't tip then the service and ability of the caddies would deteriorate pretty quickly.
As mentioned, if the skill level is so high in caddying then the cost of them should just be raised rather than relying on people to tip more.