the only time that happens is when there is an organized clan in the defense team
You can easily do it with 2 people. 1 brings the site and 1 material, 1 brings 4 material, build your weapon rack and start placing stuff. I've seen it done with 1 person but that takes longer to get going, as they have to run off and die to get more material.
Bear in mind that defense would have a use for the firebombs, too: you can firebomb incoming enemy ladders (not map ladders, but deployables, since map ladders don't take damage). And rather than make firebombs take 2 or 3 slots or be unwieldy, we could just limit their damage. I don't envision it as something 1 person brings to blow a whole door of stuff. More like you get a 1 slot, sheathable, 1 ammo fire bomb which deals, say, 50% of the health of a medium ladder in one hit (1 meter radius from where your throw lands).
Or maybe a logical thing would be that it takes 2x as many slots of firebombs as it takes slots of deployable, to destroy that object:
Small/medium ladder: 2 slots. Takes 4 firebombs (4 slots).
Siege shield: 2 slots. Takes 4 firebombs.
Siege ladder: 3 slots. Takes 6 firebombs.
Construction site: technically 8 slots (3 for site + 5 for material). Takes 16 bombs.
But the bombs have a small AE so 4 people running up with 2 bombs each will likely clear the whole door and do 50% damage to all the construction sites. Then they have to hack through the rest.
Also, for the record, I do not propose that firebombs do ANY special damage to map doors/gates.
Also, for those with concerns about realism, firebombs date back to at least the 10th century, so we are not entirely inventing a device (incidentally, if you really jammed all that wood into your keep door, the enemy would probably just set it on fire, wait, and drag your corpses out in the morning...)