Hi Campbell,
Those other sites have hundreds of developers on staff. :-) Volunteers can do a lot of things -- WikiTree is evidence of that -- but programming with volunteers has limitations. A volunteer might get excited about creating an app, but not be able to maintain it. Maintaining it could be difficult because we're always adding improvements to WikiTree.
That said, we have considered an app with limited functions, such as taking and uploading photos, e.g. for the Cemeterists.
Chris