Why something seemingly so easy is so difficult?

Why is something like pots and rules for pots or even implementing overdrafts or joint accounts seemingly so difficult for Monzo?

Just 20 minutes thought lead to 11 questions about joint accounts. Now add in Legal, Design, Usability, Ethics, Support, Data Structure, Platform…
You can rush something and improve it, or wait and present it better. But you only get a first impression once.

You can land and take off a plane with a computer but why can’t you (yet) program a pot,

Flying a plane is not easy and flight computers can be similarly complex

which is surely just a file with a number associated with it, to request a number from another file on a given date?

Except you need to guarantee it works every time, without fault or mis-step, and not run when it shouldn’t. Money mustn’t get lost if it’s a leap year or there’s an issue with the environment, it shouldn’t pay out twice on daylight saving etc.

Surely this is very easy for a computer to do?

Computers usually do what they are told, the trick is to tell them the right thing to do. And check afterwards that they have.

Genuinely interested as to the technical difficulties that must be associated with this.

People tend to not think the same as you. You could make pots and rules that work for you, but that won’t necessarily sell or scale to thousands of people.