You’re still not quite getting it.
It makes sense why the ISP can most easily measure the network bit speed, rather than the speed or bandwidth of services carried on the network.
There are two big flaws in your logic here. The first is that you’re trying to logically separate the overhead from the desired function itself such as a file download. As if the ISP should account for overhead, and give you a number that represents the ultimate file transfer speed. But both the file transfer speed and the overhead belong to you. You’re trying to get away with not counting the envelopes in the back of the truck because they’re incidental to what you actually want which is the data inside, but the envelopes are still yours, not the ISPs. “The speed or bandwidth of the services carried in the network” includes the overhead.
The second flaw is that you latched into the overhead but forgot the other half of the answer which is that you just have other traffic going on. Just because you’re downloading something from Steam, doesn’t mean you iPhone isn’t also trying to download updates, etc.
It still isn't a very useful measurement for a layperson customer. In other industries, regulations exist that make it easier for the customer to place the data shown to their real world use of the service.
It’s a very useful measurement for a layperson customer. It tells you exactly what you’re getting, and gives you a number you can use to determine how it affects your personal requirements. Sure you’d have to google it and learn what it means which will take you a few minutes, but if you really care you’ll take the time to do that. Megabits per second are your real world use of the service. There’s nothing abstract in unreal about it. Using your car analogy, does every customer know exactly what a horsepower is? Or what lb-feet of torque is? Are car manufacturers being dishonor at by not renaming these things to something every tom dick and Harry off the street would intuitively understand without so much as a google search? Of course not. And what about air conditioners? Do customers just inherently know what a BTU is, or are they expected to look into what that is and how many they need for their home before they buy one? Nearly everything you buy is like this in some level.
cars are sold with EPA-mandated miles per gallon estimates.
Yes and you’re expected to know how much liquid a gallon is and how far a mile is, then take those numbers and apply it to your personal daily commute and car usage. I don’t know why a customer should be expected to know what those two measurements are but not what their Internet bandwidth is.
You could have ISPs include a tidbit of estimated average file transfer speed, but that literally cannot be the way you advertise or bill for service. It would be informational only. And the reason they generally don’t do that is two fold, firstly, file transfer speed is not the product they’re selling you. It’s a function of a hyper specific process that is related to the product they’re selling you. Secondly, it just gives customers an inaccurate number for them to misunderstand and then complain about. When you give somebody a number they expect to see that number. It’s easier to give a customer a number that you can quantify and verify and say “Here you go, you’re getting the number we promised you” than to give them a number based on guesses and averages and then when they complain all you can tell them is “We never promised you this number, this was just a guess”. Customers really don’t want to hear that.