Really? I think it depends on a lot of things, like how many trucks are on, how much business they're getting, how much profit, how much they have put aside, etc. Look at Polly Express. I don't think they have a legitimate office after being in business for the better part of 10 years; and they're still going at it.
What's a "legitimate office"? If it contains appropriate equipment [desks, chairs, computers, phones, fax/copy machines, basketball hoop] it's an office, IMO.
Polly's 'office' may be unconventional, [and I still LMAO over the time the safety director decided to reroute the phones and crashed all the computers] but they were and are better than a lot of the 'unknown' carriers I've heard about.