As Joe-Z says, there's a large supply of cleaners available which keeps wages down. I'd add I would pay a premium for really good cleaners. A company that employs cleaners who can clean properly and who understand the ways tech workers think would be able to charge more. For example, not moving people's monitors when they clean desks...
And if you've got an office full of sensitive material, you will and do pay a significant premium over what you'd pay to clean an office full of of help-desk employees.
