I still didn't hear an explanation.

I gave several. Look down to the response I gave to gonzolegend.

ATGMs are not a superweapon. The US trained for their appearance in conventional warfare against the Soviet Union or another state power. So I'm not really sure what you're asking. If there was a massive amount of ATGMs, the reply would be to adapt to that threat environment. The leading cause of death in Iraq was IEDs. By a HEAVY margin. 4400 US troops died in Iraq. 2500 were by IEDs. So you're introducing a set of variables that just don't exist or only in limited form.

Can you really make believe that the US would just send out thousands of non-vehicle troops, in a country swarming with suicide bombers?

This is literally how the war was fought. It was relatively vehicle light until the emergence of IEDs as the number one threat. And then IEDs shifted away from troop focused and became vehicle focused. But the thousands upon thousands of ground troops conducting foot patrols never ended. I would know- I was one of those ground troops conducting foot patrols, even as part of a mechanized unit. Suicide bombers target static positions by and large. That's the greatest concentration of targets. A bunch of Marines or soldiers gathered around a central point conducting searches, talking to locals, etc etc. You know where it is, you can plan where to hit. Foot patrols are bad targets. They're mobile, they're able to displace quickly, and if a vehicle guns it towards them they shoot it. And more importantly, an SVBIED gets maybe one or two troops on a foot patrol. Everyone is spread out 10-15m from each other on the patrol (FOR THIS REASON).

To make it more obvious: what if the entire iraqi insurgency was ISIS, filled with ATGMs and suicide bomb trucks.

Why not give them a navy? Or an air force? You've thrown a shit ton of hypotheticals down that don't really have any basis in reality. But ultimately... you could replace your question with "What the entire Iraqi insurgeny was the Russians/Chinese!" and the answer would be the same: then the tactics used to defeat them would change.