Its for many reasons: 1.It scale better: less memory/rendering time. 2.It allows low-end hardware. 3.It appeals to children(probably a huge chunk of audience).

Its for many reasons: 1.It scale better: less memory/rendering time. 2.It allows low-end hardware. 3.It appeals to children(probably a huge chunk of audience). Facebook isn't going the Second Life route(visuals-first,scripting, limited capacity servers), they're making something that is essentially a casual/social videogame with VR and scripting added - and capable of hosting hundreds(like VRchat).