Not OP, but I guess it's easier because you work from left to right. At least, I find subtraction/addition easier to do from least to most significant digit, since the results of the less significant digits affect the more significant digits, but not the other way around.
EDIT: To be clear, going from left to right is not inherently 'easier' but it would be more consistent with our direction of writing.
Yes but it's cause all sorts of angst in the computer architecture world - "little endian" architectures are "pure" in the sense that arabic is with it's embedded numbers, while big-endian systems are this weird mixed artifact of our language with text bytes going from lower to higher addresses and integers going the other way.
Over time we all realised that big-endian systems (IBM, 68k, etc, and our native western languages) were a hack and we've moved to little endian systems (intel, arm, DEC, ...) as sane
EDIT: To be clear, going from left to right is not inherently 'easier' but it would be more consistent with our direction of writing.