Various reasons:
It says so on your terms ans conditions of the ticket.
The Captain has signed the tech log stating the wt. of the a/c and number of pax on board.
The cabin crew are securing the cabin and briefing the pax.
It may take 1/2 hour from shutting doors to lining up for departure.
There may be a slot delay, that ops are doing their best to bring forward, so that the plane may leave immediatly.
If you're checked in and late, your bags may already have been offloaded, and the handlers aren't going to p*ss around putting them back on again.
Your seat may already have been given to a stand-by pax.
If it's a low-cost airline, they've now got your money (no refunds!), and that from the pax who has now bought your seat.
You're big enough and old enough to get to the airport on time on your own.