I think there may also be campaign fund and council hoarding site allowances for electrical candidates, so you want to run electorate candidates wherever practical even if they don't stand a chance.
If you're going to stand ~50 electorate MPs, you might as well stick their names on the list.
Elections states that the "maximum number on the published list is 65". I'm not sure why (printing size? Practicality?) I assume that if that rule wasn't present the two major parties would be in at least the 80s.
There are 48 list seats, plus at most a few more overhang seats, and people can decline seats, so I'm guessing it's to prevent parties from putting everyone and their mum on the list since it's of very little practical use after some point.