Because, after you remove the cost of having the crew and supplying them with basic maintenance parts, it gives you an idea of how many people on the ground are needed to support each person onboard, just to supply expensive parts that are being constantly burned through.

Ideally this supply of burned-up parts would not be particularly huge.

But I didn't realize that number was for a full fifty years, so I'll take back the 'ridiculous'. It's still a whole lot.