That's why we have training; to learn those principles.
Yes, the calcs are easy, yes you could get an app or make an app, but
(a) if you don't understand the principles and input the correct figures or
(b) You don't recognise when you need to, you're going down.
That's why this Flight Instructor> United Airlines pilot > 5,750 hours > 130 hours on type slipped up.
Another classic example is this one:
The runway looks dead flat, probably just like his home strip.
Why would you do the calcs here?
Would you have check the strip for height at the takeoff end, then the end of strip, start of treeline against the aircraft capability?
Maybe spent some time in a climbing circle directly above the strip to provide a forced landing point?
The pilot here didn't just have 1, 2, 3 No Go points - he had about seven - the longest failure to act that I've seen.
You won't get that on an App.