What does a government do that a company can also not do, theoretically? I think that government is an arbitrary label.
"Legitimately" use aggressive force.
A company could also use force that would evoke quotations around "legitimately."
Of course they could. But everyone would see those quotations. If a government uses aggressive force (say, to enforce its laws) then everyone is OK with that, because it's a government. If a company uses aggressive force to enforce its edicts, then people tend to have a problem with that.
Government = some humans wielding great power. Anarcho-Capitalism = all humans wielding great power.
Fixed that for you.