The problem is that it does require manual intervention on reboot and may not be very useful for 100s of machines.
Actually even right now I hope the server has been rebuilt from scratch along with grub and everything. If they just restored the server as it was left, for all we know they could be running a rootkit or a hacked version of bitcoin daemon.
They have gates and retina scans and cameras, but they have these things specifically because many strange and mysterious people are allowed in very close proximity to your servers: It is a compensation for the greater risk that the environment naturally presents, rather than being above and beyond the normal security.
But it does nothing to defend the physical security when security is theater (which it almost always is, and where social engineering easily circumvents it), or when employees of the data center are a potential threat, as are the many other customers of the data center are the threat.
People can get on aircraft and through international borders with assumed identities. They can certainly get into a data center, especially given that customer service always prioritizes convenience ("Something must be wrong with your retina scanner. Anyways I really have to get to that server now". In most situations, you'll be getting in).