Factoring numbers is difficult. Manipulating digits is easy, or it looks that way, but this is partially an illusion resulting from the number being given to you with the digits already known.
If you have a quantity in mind but you're not sure what its digits are, it can be a lot of work to learn.
Maybe it's an unfair example but: the number of people on Earth. We know the quantity more or less, the leftmost digit in base ten, then it's a lot of work to figure out the other digits.
If you have a quantity in mind but you're not sure what its digits are, it can be a lot of work to learn.