If you hand wash and line dry, suppose that's 5 minutes of labor per wearing. If you wear the shirt every week for 10 years, that's 43 hours of labor on washing for 10 years worth of shirt. It would only take 25 hours of labor (compare to 48) to buy a new shirt every 2 years.
And this also assumes your time is equally valuable to the time of the guy making the shirt - I consider my time to be worth far more than $5-15/day.