The equipment could be mobile or portable
Have you seen those baggage x-ray machines? It would take a 2+1/2 ton truck to move one.
Take an airport that runs 60 flights an hour. Under the present centralized system gates are open about half an hour so we'll say you need 30 gate agents. But we've only got 10 security, baggage and immigration staff because everyone comes to them over a period of time before the flight. Under your plan the gate would have to be open (and staffed) much longer to complete all the events at the gate. Boarding is just 15-20 seconds flashing your boarding pass under the scanner. Checking your bags (and collecting the fee), security and immigration each take minutes. I say it could take an hour to process 200 people. So now you need 60 of each of those job titles. That's not saving any man hours.