This is in practice not true at all. Vertical scaling is typically a sublinear cost increase (up to a point, but that point is a ridiculous beast of a machine), since you're (typically) upgrading just the CPU and/or just the RAM or just the storage; not all of them at once.
There are instances where you can get nearly 10x the machine for 2x the cost.
Given what engineers at this level cost, their costs per hour dealing with all of the nonsense clouds handle for you (networking, storage, elastic scaling, instant replacement of faulty servers, load balancing, yadda yadda) end up being higher than whatever tax you're paying for using the cloud.
Economies of scale are real.