It’s because computers use binary and binary can’t accurately express decimals, so for money computers store the pounds and pence separately as two different values usually - can’t have money going missing to getting created because a computer can’t express it!

When you use a visual interface it’s pretty easy to get you to put in a dot or enter two fields or whatnot and you can see it, but on the phone it’s harder both to get you to do the right thing and have you check it, that’s why they often ask for you to input in pence. They don’t have to of course, it’s just whatever that company thinks will cause the least amount of human error