Why are service/delivery vehicles

[Marthe48]

parking in the street? They are parking in the traffic lanes, where on-street parking isn't available. This happens where I live and happens where my friend lives. The vehicles block traffic, hold traffic up. Depending on if it is delivery or lawn service, the vehicles remain parked for minutes to an hour. I thought it was illegal for vehicles to park in the road. Most of the houses they park in front of have drives.

[TwilightZone]

1. "where on-street parking isn't available"

There's your answer. If there's nowhere to park, the remaining option is to park in front of the place of delivery while the delivery is being made. That's happened in cities I've lived and visited for as long as I can remember.

It may be inconvenient, but I'm not sure what other options they have in some cases.

[Marthe48]

2. The driveway

Usually there is a driveway with room to park right beside the vehicle parked in the road. It doesn't take much time to pull in, deliver, pull out. I don't have a beef with the situation you describe.
A few weeks ago, a UPS van parked in such a way that if I had tried to go around, I would've hit a mailbox on the other side.
I don't think drivers in motion should be blocked.

[quaint]

3. Several workers have explained to me that they are not allowed to park on private property.

Amazon dude said they can't even use the driveway to turn around.
My but-but safer has gotten me no where. I try not to have anything going on trash pick-up days.

[WestMichRad]

4. Well not all their drivers have gotten that memo

… some of them turn around in our dive, not an easy thing with a delivery van as the drive is narrow and bordered by deep culvert on both sides. Like the FedEx driver who parked in our dive for several minutes yesterday. (I wondered if he was napping…)

[quaint]

5. Apologies, I neglected to account for difference in Mich and Calif insurance and laws.

[WestMichRad]

6. No apology needed!

Delivery vans do often park in the road here, too. But not always.