Are you seriously claiming that everyone who can install a diet app is also able to successfully permanently modify their diet with no additional assistance?

Yes. Being able to do something isn't the same as actually doing it though.

Being physically able to do something isn't the same as actually being able to do it. Mental/emotional contexts can make it very hard/stressful to change your eating habits, and often the stress isn't worth the end result.

This suggests a way to make changing habits more convenient, and the response of some people here is to sneer at the people who need such convenience. Which is somewhat typical, but still unfortunate.