You pay for the scalability. Your 4 core AMD is always a 4 core AMD unless you migrate to a new server. Your 1 core Linode can become a 20 core Linode in a few minutes, for just a few hours while you need the capacity, and then go back to being a cheap 1 core Linode.
And one server will only ever be one server. For many deployments it makes more sense to have several smaller nodes distributed between different geographical areas than one big node in one datacenter. And/or multiple nodes in a a single datacenter so you can perform rolling upgrades.
