SimAuckland -- Rail to the Airport

I've seen a lot of talk at different times about rail to Auckland Airport. The proposals all appear to be either extending the Onehunga line through Mangere, or head out the back to Manukau.

I've never understood why a link directly across to Otahuhu is not proposed as an option. There are few streets to cross, and not that many properties that would need to be bought. In fact not so long ago the path the line would've run through appears to have been vacant, almost like it was reserved for a road or something.

I also think this has the nice affect of allowing transfers between more lines (see second image).

Is the area too lumpy or something?