We manage upwards of 30 different compute clusters (many listed here: https://hpc.llnl.gov/hardware/platforms). You can read about the machine slated to hit the floor in 2022/2023 here: https://www.llnl.gov/news/llnl-and-hpe-partner-amd-el-capita....
All the machines are highly utilized, and they have fast Infiniband/OmniPath networks that you simply cannot get in the cloud. For our workloads on "commodity" x86_64/no-GPU clusters, we pay 1/3 or less the cost of what you'd pay for equivalent cloud nodes, and for the really high end systems like Sierra, with NVIDIA GPUs and Power9's, we pay far less than that over the life of the machine.
The way machines are procured here is different from what smaller shops might be used to. For example, the El Capitan machine mentioned above was procured via the CORAL-2 collaboration with 2 other national labs (ANL and ORNL). We write a 100+ page statement of work describing what the machine must do, and we release a set of benchmarks characterizing our workload. Vendors submit proposals for how they could meet our requirements, along with performance numbers and test results for the benchmarks. Then we pick the best proposal. We do something similar with LANL and SNL for the so-called commodity clusters (see https://hpc.llnl.gov/cts-2-rfi for the latest one). As part of these processes, we learn a lot about what vendors are planning to offer 5 years out, so we're not picking off the shelf stuff -- we're getting large volumes of the latest hardware.
In addition to the cost savings from running on-prem, it's our job to stay on the bleeding edge, and I'm not sure how we would do that without working with vendors through these procurements and running our own systems.