One reason that it is more than one, could be to show novice users that it is possible to have more than one.
If it only gave you one sheet, I suspect many people would never progress beyond single sheet workbooks.
Having said that, I have set mine to 1 by default, and prefer to add sheets as I need them. It always seems to me a bit "wasteful" to receive a spreadsheet from someone else that contains a few entries on sheet 1, and then empty sheets 2 and 3. It can also be a little time consuming, if you have to check them to see if there is anything on them.