Why we always split into ten

Each time we needed more accuracy, we split into 10. Couldn't we split into 4, 5 or 8 instead?

We could! The same length can be shown with the unit split into 10 parts or into 4 (quarters). For more precision, split each quarter into 4 again: parts of 1/16 of a unit. Nothing wrong with that.

So why always 10? Because of how our place value system works. In 281: 2 hundreds, 8 tens, 1 one. Each place is 10 times bigger than the place to its right, and 10 times smaller than the place to its left. 10 ones = 1 ten, 10 tens = 1 hundred, and so on.

To go below one, we just keep the same rule going to the right:

It carries on for ever, just as the places to the left keep getting bigger.

This way of writing numbers is the decimal system, because it is built on ten. Decem means ten in Latin, related to the Sanskrit daśha, and to the words for ten in Odia, Konkani, Marathi, Gujarati, Hindi, Kashmiri, Bodo and Assamese.