To add to the other (good) answers: it is also because this makes (re)starting the engines in zero gravity easier. Liquids behave very weird in 0g, trying to su

To add to the other (good) answers: it is also because this makes (re)starting the engines in zero gravity easier. Liquids behave very weird in 0g, trying to suck fuel from a nearly empty tank in 0g can be challenging. Leaving more fuel in the tanks makes this less of a challenge.