This is the sunk cost fallacy. While you've overlooked it, I promise you they haven't. If they already have the lease, they'd be happy to let the building sit empty without paying for utilities, security, etc. They'd be happy to offload facilities costs to employees.
If they could save money right now by not paying for utilities and support staff without sacrificing productivity, they absolutely would do it.
And eventually that lease is going to come up for renewal, and they'd love to be able to cut costs then too. Bringing everyone back to the office makes that impossible, but they've decided it's worth it.
Things like utilities and support staff on large business park leases are often included or heavily discounted, not to mention that they're usually tax-deductible. Where are you getting your info?
EDIT: And if they DO own it, it they would probably prefer to pay some rental management company to rent it out as office space for passive income rather than fill it with their own expensive employees.
A small part may even be that they value work done in-office more than work done remotely. But just pointing out it’s not the most plausible objection to remote work.