I suppose Intuit, just like other major professional tax software developers, is spending their resources on developing tax programs and other features that tax professionals need in a conventional setting.  There is not an app for PTO because it would be unusual for tax professionals to prepare returns and manage their portals over a mobile device.

As mentioned previously, PTO can be used on a mobile device but it is not recommended as the rendering may not work as designed and certain features may not be accessible.  So, it is your decision whether to use it on a mobile device but YMMV.

You will see that Link for taxpayer has no app either.  But that is because Intuit has improved Link for taxpayers as a responsive and mobile-friendly website over the years, so that it functions the same way on both mobile devices and computers.  The app they had a few years ago was retired as a result.