Hetzner is not only able to get better volume discounts, they also spend a lot of engineering time bringing down costs, and have decades of experience doing so.

They offer some Dell Servers for those that really want them, but most of their servers have a custom mix of consumer hardware, server hardware and in-house hardware (for example they use their own racking system), optimized to minimize lifetime cost in a datacenter. For example most servers use datacenter SSDs, but consumer CPUs.