What is an SI base unit for measuring length?
SI unit of length is "meter".
No, although a centimetre is a unit of length, the meter (m) is the SI base unit for length. 1 cm = 0.01m. Not only isn't it a base unit, it's not even an SI unit! In the SI system, multiples are based on 10 raised to the power of multiples of 3. So, whereas a millimetre, is an SI unit, a centimetre is not -although, of course, it is a metric unit.
Quite simply, because that's the way the SI was defined. It really depends on the choice of base units - for the SI, the meter (a unit of length) was defined as the base unit; meter cubed is the derived unit for volume. A liter is 1/1000 of that, and is therefore also derived from the meter. You can just as well invent a unit of measures in which a certain volume is the base…