First, changing out the CPU type to something that isn’t almost identical out from under a running VM is a mess and may require degrading the system by removing features from the starting CPU. Switching manufacturers at runtime (AMD vs Intel), while sort of possible in theory, is effectively a lost cause.

To add CPUs, you have to deal with the architecture’s nasty hot-add CPU mechanism, and the guest OS needs to be expecting those CPUs starting from when it boots, and that expectation isn’t free. (The latter is Linux’s num_possible_cpus vs num_present_cpus.)

Adding memory involves getting that memory into the kernel’s memory map in the right places. This is more complex than one would like. Removing memory is worse.

And all this happens, in the cloud data center, on essentially normal hardware. If you want to add RAM, the CPUs need to be on a system with more RAM, preferably on the same NUMA node. And vice versa for adding CPUs. If the tenant is paying for local storage, that needs to move, too.