Besides the other answers, could also be that they wanted to make sure they had their hands on a fully deterministic implementation across different hardware/so

Besides the other answers, could also be that they wanted to make sure they had their hands on a fully deterministic implementation across different hardware/software stacks (like Windows and OS X). As a sidenote on a project the size of LoL, the time, size and cost of implementing a PRNG based on a known one is a rounding error.