North York Centre was an infill station, an infill station that was planned for a long time, and tunnels were built with it in mind. The actual extension to Finch was a multi phase project that extended the line from Eglinton (first to York Mills, then to Finch).
The only 1 stop extensions were A) Wilson to Sheppard West (this was built so that the subway could meet up with the future Sheppard Line), and B) Kennedy and Kipling, which were made to provide a connection to future suburban LRTs, not to mention both of those extensions were above ground/cut and cover under a former rail ROW, so they were cheap to make. Extending the line to just Steeles would likely involve buying a TBM just to use it for 1 stop (granted it shouldn't have been but oh well).