Same reason that the Portland & Western leased the track from Springbrook to Sherwood over Rex Hill (that has exactly ZERO customers, and currently is not in use). The railroad has the right to run trains over it, but operationally has found a better way to do it. P&W runs its trains over the W&P leased lines from Newberg south to Albany; CORP runs its trains north to Eugene.

Willamette Valley Railway had the Geer Branch at its inception; the track has since been abandoned (save for a short stretch at its east end for car storage); the WVR now runs all trains north to Woodburn for interchange. The P&W also leased the route over Cornelius Pass between Banks and Linnton even through it very well could run the trains through downtown Portland saving about 50 miles (however due to track congestion it runs log trains over the hill.) P&W also leases the line all the way out to Astoria but hasn't run a train out there in years. W&P has the old Westside Line between St. Joseph and halfway to Carlton yet the track is there only for car storage and nothing else; same with the old St. Marys Lead in Beaverton (the old connection between the SP and OE from the mid-1980s until the mid-1990s when MAX took over the OE from Beaverton to Hillsboro); now that the GM warehouse is closed, the lead still exists - again, for car storage.