I mean, you're supposed to be at the gate when boarding starts. For all practical purposes if you miss boarding you've missed your flight even if the plane hasn't left the ground due to safety rules about letting passengers go after the door is considered closed.

There's a bit of a checklist of things that need to be completed after the door gets closed and the gate pulls away. They're often incentivized to try and speed things up, so if there's nobody at the gate anymore to board they might assume those passengers decided not to go on the flight. Especially with people now checking into flights the day before, the airline may not even know if the passenger is even at the airport and would realistically show up to the gate anytime soon as checking in means pretty much nothing these days.

They're not scamming anyone. Your ticket said you were supposed to be at the gate to board at a certain time. You weren't there, so you missed the flight.