Spreadsheets are set up this way because 99.99% of all spreadsheets are a lot like the one in the short example above: Very repetitive data, all of which you want to handle in basically the same way.
Can you imagine if you had 200 salespeople and had to type in all of those formulas by hand? Forget it! -- you might as well still be using paper and a calculator!
By using relative cell addressing, you can fill in those 199 extra cells simply and easily.