NONE of the towing services own a single wrecker.
They hire contractors, and often the contractor works with several tow services. Mostly they are independent or small businesses.
Where and when are big factors.
I-95 on a holiday weekend, it will be a long wait because the actual tow trucks are busy, and as mentioned, they get to the higher paying jobs as a priority.
If you breakdown on I-10 in far west Texas, it is going to be a long time. I've seen a truck that had to drive over 150 miles one way to get to a service call.
But how the towing service deals with that on the phone is a factor. We've been happy with Good Sam Platinum the five or six times we've used them over the past 10+ years.
VERY IMPORTANT
Sign up for the HIGHEST level of service they have. It does make a difference.