A colocated rack is a much more limited concept than a VPC. VPCs let your architect an entire network. You can have multiple private and public subnets, set security groups to filter traffic between them, do service discovery, use policy based access control, health check load balance, and host PaaS entities into the network (like Aurora serverless). On top of that, you can flex your compute. VPC is more like a rack with a firewall, an f5, a smart switch with vlans, something like kubernetes to automatically scale compute… but there are things that aren’t even possible in a rack because you can transparently both manage your own compute with ec2 and add PaaS managed offerings like RDS, elasticsearch, kafka, etc. all to the same network.