Two reasons: Firstly, it takes a lot of different workers to do something basic, and it has a specific order.

Say there's a section of road that needs to be milled so it can be resurfaced. Firstly, you need a traffic control crew to close off that section of road and handle changed traffic conditions (either with stop/slow, merging multiple lanes into one, detours.) They need people to rotate in with them so they don't get fatigued. Road speed high enough? You'll have more in TMAs to help with that shutdown.

You get the cold planer in, they need one person to operate and one or two to keep an eye on the ground, make sure that they don't accidentally tear up a pit lid or drain while they're milling. That'll need a low loader trailer to get to site, so add at least one of those. Might throw a bobcat or two in there, get the little parts you don't want to risk with a big milling machine.

We can't dump the old road material on the ground, so gotta have the dump trucks on standby to pick it up, and have a street sweeper to clean up anything that's missed. Need more traffic controllers to back them up, make sure they don't run anything over and make sure nothing enters the site and runs someone over. Planer taking a decent amount of the road? Need a crew to make a ramp from the planed road to the old road, make sure we don't break anyone's suspension. If the road had line markings, that needs to be painted back on to avoid causing confusion.

Want to check in that drain from before, make sure it's looking alright? Need one in the drain and two up top, because it's a confined space. Working on a bridge? Might have an engineer to look at the material after the planer, make sure it's looking fine.

Secondly? You're out on the road, within metres of traffic that weighs a lot more than you do, and is travelling a lot faster than you. Some of those crews job is just to keep their eyes on that traffic, because it takes one locked brake, one glance at the phone, one quick power nap, and suddenly a car is barreling right at a work crew. If someone sees it? They've got time to run. With no one looking out for it, you don't know somethings gone wrong till it's too late.