Ask HN: Would it cost less for a large scale company to use mainframes?

Very large scale companies, such as Google and Amazon for example, relies on hardware servers infrastructure to support the running of their services.

It is in my understanding that these companies use x86 and ARM based rack-mounted servers pushing them to scale horizontally rather than vertically.

Why don't these companies use mainframes (IBM System Z for example)?

Wouldn't they benefit from the following advantages? - Simpler software architecture that leverage vertical scale - maybe less datacenter space required? - maybe more efficient power consumption?