Your friend doesn't have to do it because that's not his job. He's a booking agent, not a tour manager. Big difference.
If a band can hire a sound, light, and back line company, fine. However, the band might still have 3 keyboards, multiple instruments, like saxophones and guitars, drums, computer units, stands, clothes, and other things that the back line or sound company doesn't provide. All these things have to be torn down at night, and they travel with the band. Sometimes the drum kit is rented, but most drummers still carry their sticks and cymbals.
Additionally, if you use a rental company, you have to be at the venue even earlier, and trust that the people running the show/ sound will actually know what they are doing. Often, they do not. Or equipment is missing, not compatible, or set up incorrectly. Or they are incompetent. That's why bands put weird things in their riders, so they know right away if the venue and promoter read the contract. Bowl of all brown M & M's? Yep, they read the contract.
Not all rental companies are bad. But some are horrific. I saw an employee of a rental company one time actually UNPLUG the keyboard of the opening act in mid-song. The rental company had broken the keyboard for the main act, and the main stage was a 1/4 mile away. They didn't have a backup, (although they had 6 hours to locate one) and so they told the opening act that they would need it as soon as the last song was finished.
The tech didn't want to wait. So, in the middle of the last song, during a sax solo, he unplugged the keyboard, making a loud pop, and the head guy then turned off the power to the rest of the band. The tech walked onto the stage and took the keyboard off of the stand. In front of thousands of people. It wasn't my guys, but it was a friend of mine. He is an internationally known artist, and he was PISSED.
I'm not saying that this happens all the time. There are a lot of good companies out there. But there are often issues that concert-goers know nothing about. It's stressful.
And smaller touring bands do have roadies and coordinators. I've been one, so please don't tell me that they don't. They often need someone to run interference, get things set up, plan travel, etc., so they can perform. I have been the coordinator for several tours, including one that went worldwide, and where the band played every day for an entire month, across three continents. It went perfectly, because I planned everything.
As I said, I do not want to give personal info, but it isn't like the musicians walk in and everything is perfect. That only happens with the BIGGEST stars, and even then, it isn't as glamorous as it seems. I have a HUGE amount of experience in this field. I've kind of seen one of each, lol.
So, your experience is yours, mine is mine. I'm not saying you are wrong, just that you have not experienced what I have.
And, incidentally, why do you volunteer? They should pay you.