I do it because I enjoy it. There are more good days than bad days and if you can learn from the bad days you will soon have some more good days.
I think that racing has more enjoyment when you sail for a whole season and can experience genuine improvement. It certainly works that way for me. I often say to beginners that it is a significant challenge to race for a whole season and to finish most of the races. At first they are a bit offended as they think that finishing is easy and that they want results. As the season goes on they realise that it is hard (for a beginner) and that they are improving and they enjoy this improvement.
I too have seen people spend lots of money, get poor results and drop out. If you set the right goals you get improvement and enjoyment. If you set the wrong goals you get frustrated and drop out.