[lazysheepherd]

Consider all 4 dimensions of the battlefield. You cannot be aware of every direction, at all times. Especially in cities, which has 3 spatial dimensions of potential hostile infantry positions. Therefore sheer numbers do matter.

Especially in the cities, all insurgents have to do is wait, and rain down bullets. Many will miss, but some will hit.

[giantrobot]

One key to success in military operations is tempo. Armed defenders using guerrilla tactics disrupt an attacker's tempo. Every minute a platoon of soldiers hesitates crossing a street or gets bogged down clearing a building is a minute closer to sunset, a minute more of vehicles burning fuel, a minute longer for defenders to get reinforcements, and more stress and fatigue on the whole unit.

An attacking force doesn't have infinite resources to attack indefinitely. A mechanized force that runs out of fuel is extremely vulnerable. Those extra minutes burning gas not being able to push forward add up.