Getting a USDOT number is registration, not permission to haul for hire. It's what shows up in FMCSA's Company Census File the day a company registers, and it's the earliest public signal that a new trucking business exists.
A separate, additional credential — an MC number — is required on top of a USDOT number for carriers that haul for hire across state lines. Plenty of USDOT-registered carriers never need one, because they only ever haul their own goods or operate under an exemption.