Usually what keeps people out of shelters is that they are not staffed/resourced enough to maintain safety. One person being violent can ruin the resource for everyone else. Poor access to health care and hygiene also can destroy a shelter's utility.

Most people don't want to get assaulted, robbed, or infected with disease/pests.

That is a reason why empty office space doesn't solve anything. There's already plenty of of it without Twitter HQ. Running a temporary shelter is more about the logistics/support than it is the physical space.

It’s impossible to provide adequate staffing without restricting what people can bring, what people can do, etc.

Someone with a knife requires 4-5 people to subdue minimum, etc.

Someone with an out of control dog will cause serious, potentially fatal injuries before anyone can react.

That people often get these things and want them to protect themselves just makes the problem more intractable, because when you add drugs and mental illness into the mix, the people are impossible to predict too.

Empty office space definitely doesn’t solve it. But doubling staffing at the homeless shelters won’t either.