If you only need one VPS, then you potentially only need a tiny fraction of 1U worth of compute/storage. That’s not a sensible colo use case.
From the DC perspective, the biggest costs for providing colo are power, AC (which is mostly power), network and real estate. Supervising rack access is a very small line item in their accounts.
Supervising rack access and/or using physical barriers was one of a list of different reasons for why the cost per 1U is so different if you buy 1U rather than a full rack. It may not be the most significant one, but it is there.
As for power, and network, it's often charged separately, and you will still find the 1U vs. full rack difference then. Sure, you can to some extent perhaps assume a slightly lower load factor for customers that rent a full rack, and that may contribute too.
But the point remains: The person above me should not be surprised that renting space by the 1U slot is expensive.
From the DC perspective, the biggest costs for providing colo are power, AC (which is mostly power), network and real estate. Supervising rack access is a very small line item in their accounts.