Yes. Volunteers fixing potholes seems fine.
But when about when a bridge needs replacing? I want to know that someone is following standard engineering and construction practices and that they're accountable to do so. I don't want to learn about Jimmy's Artisinal Welding by plunging to my death.
There's also no good reason to believe that anarchists wouldn't follow standard engineering and construction practices. Look at open source software, there's no state in control of practices there yet engineers tend to do just fine. In fact, there are plenty of examples where people cut corners on safety procedures within a capitalist society in order that they can get a competitive advantage in the market. I'd say the incentives of capitalism are more detrimental to good bridges than those of anarchism.
I don't know why you think that a different organizational scheme would prevent corruption.
If you're going to build a bridge, somebody has to do the work. Everybody else is going to pay them somehow. And if you're going to pick someone to do the work, somebody has to choose. It's untenable to have every decision made by all people, so everyone will delegate, say, construction hiring decisions, to someone.
Now the Construction Hiring Decider can pick who gets paid to work. Who will he/she choose? Will it be based on skill, personal connections, ethnic bias - or maybe kickbacks?
You can't run any large group without delegating some responsibilities. As soon as anybody has any decision-making power delegated to them, they can misuse it. There is no system that can totally prevent that. And the more structures of accountability and shared decision-making you build to safeguard power, the more it looks like a democratic government.
You do raise good points but anarchists are not all juvenile and naive as many think and, seeing as anarchism has been around for >100 years, people have thought of these issues before you. Decision-making definitely has to be delegated to people sometimes and the manner in which this is done is important. Usually anarchists emphasise frequent rotation and possibility for immediate recall in order that no one may abuse their power.
I think of it as being similar to how science is funded by national agencies. There is a big pool of resources and it gets dispersed among researchers according to some process. That process usually involves nominating a team of scientists in the relevant field to judge where the resources would be best spent. Everyone ultimately has the same goal of advancing research most effectively and so they usually make the best choices possible. The panel is different every year so there it's hard to have collusion going on. Academia definitely has its problems with entrenched hierarchies of course but those come about for different reasons.
It's impossible to make a perfect system but it's clear we could do better. Anarchists just want to dilute unjust power structures as much as possible. There are examples of organisations that are successfully run like this so it's not like these are utopian ideas.
A very large fraction of open source software is crap. Fortunately we get to choose which OSS programs we use. Not so for bridges.
There's also then no accountability for when it fails. Not that we have that now, necessarily, in software "engineering", but in other engineering disciplines, we do.
A licensed engineer can lose their license (a huge professional blow) and be potentially civilly and/or criminally held liable for designs they signed off on that fail or violate appropriate ethical standards of their governing body, e.g. IEEE for electrical engineers.
Eventually they might even make it official by referring to those people as a "government".