• Extremely time consuming to build. Phabricator is 2 people, so building a mobile application would mean no other features would get built for years.
• No real interest from users. Most people who ask for this feature are new to Phabricator.
• It's responsively designed. Phabricator already works on mobile devices through responsive CSS, even code review.
• No benefit. We spend time building features that solve problems. Since we already have everything available through responsive web, no new functionality is gained by offering a dedicated mobile app.
• It would be Phacility limited. Building an app only makes sense for Phacility, since it's open, and your local install is likely not accessible from the open web.
• Most other competitors do not offer native apps, and I'm not aware of any code review product that has a native application.

(Responding to the closed question, to get an actionable answer into it for future searchers.)

Most of the time when I see questions like this, the real question is "how can I make it easier for users to get to the thing when they may not have a keyboard?"

The easiest way to ensure your users have a Phabricator icon on their devices, honestly, is to use your MDM (mobile device management) tools to push your Phabricator URL with an icon to your users' devices. That will allow one-tap access to your Phabricator, though it will be in their web browser.

This is, of course, assuming that your MDM is properly configured to establish any needed VPN connections (either always-on, as-needed, or on-demand), but that is well outside the scope of Phabricator.

This solution isn't really suitable for smaller organizations using Phabricator who do not yet have centralized management of their mobile devices. But for those who do, this is as close as you'll get to a "native" experience without building your own app which just wraps your Phabricator in a Web View. This is less effort and easier on your users.