Because building a multiplayer mounting system is a lot more complicated than you think. There are a plethora of problems to solve when building what should be a basic mounting system let alone one that is online multiplayer friendly. This plugin helps you add Vehicle and Animal mounting to your game with just a small amount of setup but also a high degree of customization in behavior. It comes fully network gameplay supported for multiplayer games as well as custom mounting and dismounting animation support. It also allows you to create technically unlimited seats per vehicle and mount so you can have any number of passengers ride along with you. Finally it fully supports turrets with it's Linked Actor feature.
Find it on the Unreal Engine Market Place Pro Mounting System
Release Notes v1.5