both of them,为什么不是both of they?

因为“of”是介词,“them”是“they”的宾格,介词要加宾语,也就是“介宾”形式,所以“both of”后要加“them”,不能加“they”.

扩展阅读：bothof us together ... all of the above ... two of whom ... away from ... both of them like ... both of them are ... both of them pass it ... both and ... all of them are ...