What's the big deal with late cancellations/no shows?

It's part courtesy, extra work and team fairness. The team formats favored by the club require matching golfers and teams to create a fair and fun tournament. A change after the line-up is set requires the Tournament Chairman to rerun all golfers through the matching process. (How are tee times arranged?) If done less than a week before the tournament, it also requires changing the website and the printed materials created for committee members, course marshals, etc. If done at the course, then it compromises the outcome of the tournament by putting teams at a disadvantage.

Unexpected events happen when members can't play. Injuries and work commitments do come up. But they should be the rare exception.  See: When Do I Have to Cancel to Avoid Paying Penalties? and What Are the Late/No Show Penalty Fees?