AngelList isn't using this money for their own operations, they're using it to invest in hundreds of other startups. You question is equivalent to asking "Why

AngelList isn't using this money for their own operations, they're using it to invest in hundreds of other startups. You question is equivalent to asking "Why does KPCB need so much cash?" to which the obvious answer is "so they can invest in interesting companies where are likely to generate substantial returns for their partners."