A game engine has to be pretty popular to have lots of 3rd party ready-made assets. In most of game engines, you have to code and then create your own assets. An engine just has to provide basic framework and toolsets which MV does & has. What MV has in 3rd party ready-made assets are very little compared to what Unity marketplace has. But then Unity is a lot more complex. If you've ever worked with Unity, you will find prices of the extra packs here relatively cheap.