Because building a multiplayer mounting system is a lot more complicated than you think. This plugin helps you add Vehicle and Animal mounting to your game with easy setup but also a high degree of customization in behavior. It comes fully network supported for multiplayer games as well as custom mounting and dismounting animation support. It also allows you to create multiple seats per vehicle and mount so you can have passengers ride along with you.
Find it on the Unreal Engine Market Place Pro Mounting System
Release Notes v1.2