Why hasn't the layer 2 domain been phased out of Data Centers?

recently started working as an Onsite Engineer.

My boss was showing me around one of the data centers in our building and explained why we had a few empty racks with patch panels and all the copper completely cut. He was telling me how they have transitioned from a tier 3 network architecture to a "Spine and Leaf". From my understanding Copper patch panels are generally capped at 10gigs/port. In addition to having to run STP on your layer 2 switches you are sort of forced to lose bandwidth for added redundancy in order to prevent loops within the layer 2 domain. Why hasn't everyone stopped using layer 2? It seems alot smarter to just have everything being routed at layer 3 for maximum bandwidth. You can still have redundant links and you won't be limited to only having one route throughout the domain.