Nobody claimed it was hugely profitable, that's just a straw man, which along with making up numbers seems to be all you have.
However in the right place, under the right circumstances, they can be very profitable, it all depends on a bunch of circumstances that would have to be investigated before you invest in building/buying the stadium. BTW, a lot of stadiums overseas are privately owned, particularly in Europe.
Besides, as has been stated multiple times now the value in the NRL owning a stadium isn't in the stadium it's self, but in owning the land. The land is the valuable asset, the stadium just justifies the NRL owning it and puts it to good use for the sport until they need it.
Why Wellington (or somewhere similar), well precisely for the reason you condemned it for.
It's a smaller city where good land is cheaper, but it's growing quickly which means the value of the land will grow quickly. There's no competition from other large rectangular stadiums, and there're multiple pro teams that could be permanent tenants. In other words you'd effectively have a monopoly on rectangular stadiums in a market with at least three teams to use it, and that's a very unusual set of circumstances.
Your problem is you are a small minded moron condemning things before you've even thought them through.