Most of them run Debian (some have Windows VMs running on those Debian machines), while a minority use Ubuntu. I reboot them once every few years when I upgrade the OS, kernel, or migrate to newer machine types.
I run most of the workloads in containers, but there are also some VMs (mostly Windows) and some workloads use Firecracker micro vms in containers.
A small number of machines are rebooted more often because they occasionally need new kernel features, and their workloads aren't VM friendly, so they run on bare metal.
You did not say what system you use on them, but don't you need to reboot them to apply kernel upgrades, for instance?