Cloud providers have to make money, though. You're doing more with bare metal, yes, but you're not doing much more (especially compared to learning how to use a cloud infrastructure effectively), and the cost may not be worthwhile. Generally, the cost of buying and running one server in a datacenter and staffing the ops team needed to keep the hardware running is cheaper than paying for a 24/7 server of comparable specs in the cloud. And if you're already running in a datacenter, it's extremely not obvious that the cost of moving your workload ever will be net profitable.

Of course, if you don't need comparable specs to an entire physical server (I do a lot of personal stuff on a t2.micro!) or don't need the same capacity 24/7, the math is very different. The cloud is definitely cost-effective for elastic workloads.