I think that the way these instances are classified is based on the bare metal they run on. So for instance an r4.xlarge is a single VM in an r4.metal server ac

I think that the way these instances are classified is based on the bare metal they run on. So for instance an r4.xlarge is a single VM in an r4.metal server acting as a host. So only r4 virtual instances (r4.xlarge, r4.2xlarge, etc...) divide up nicely on these r4.metal hosts. (RAM, CPU, Instance Stores, etc...) If they made fully customizable instances then they could be using 100% of the host machine's RAM, CPU or whatever but only a fraction of the other factors. This causes a metal instance to be occupied entirely by a single VM which would make it cost a TON! So in order to make customizable instances they would have to make a ton of differently shaped metal instances to match all the possible types of custom instances and make a best fit. In the end they end up with what they have now anyways. What would be cool is if Amazon would let you order custom hardware for bare metal to go in their racks. They acquire and set up the server and load their host image. (Sort of an EC2 Colocation service.) Then let you divide it up as you see fit into virtual machines. Sort of a (Build your own instance class) thing.