It also is worth pointing out that it's not just because of the "rarity" of goalies teammates do this - it's also a substantial cost burden for a goalie to play due to the expense of equipment. This helps distribute that burden a bit more fairly since as you stated early - the team doesn't exist unless you have a goalie.

A family member recently outfitted a junior high goalie and the total came to about $1,000.

I always imagined the goalie as the main tank of a raiding guild. They got special privileges just due to their importance and "upkeep" costs.