So, let's think back to the analog, circuit-switched telephone network. If you have a telephone line, you generally expect to be able to make a call at any time. But for anyone to be able to make a call at any time across the country, you would need to have one cross-country line for each person on the side with lower number of people (assuming, of course, that you divide the country in half and need to run cross country lines between the two halve; bear with me, this is a bit of a simplified example).
Of course, if you built one line for each person, then the vast majority of them would go unused. It would be completely ridiculous to overbuild like this; everyone would be paying for a ton of capacity that's unused. So how about if they build it to comfortably cover 10% more capacity than they've ever had used in the past. This allows for enough headroom for spikes, and when usage starts crossing that line they know that they need to start building out more capacity.
But of course, then some disaster strikes, and everyone tries to call their loved ones at once to check in on them. Usage surges to more than 10% higher than their previous high, and they run out of lines. Oops, now they've violated their "minimum service level". So, should they only have sold you long-distance service when they knew that there was less than one line per endpoint? Well, sure, it would have been ridiculously expensive and you wouldn't have bought it to have your own dedicated line the whole way across the country.
Now, the considerations in a packet switched network aren't all that much different. You can be more efficient about sharing the same resources because you can even share a single line, with several people sending packets at different times. But if you want to provide some kind of guaranteed minimum service, you need to take whatever you most limited path is, and divide up its cost and bandwidth between all customers who could possibly use it simultaneously. And I guarantee you, that will add up to paying a fortune for a pittance of bandwidth.
Why should it be acceptable to say, "How dare you
maximize your use of this, when it reduces availability
for everyone else?!?" Nobody gets a guarantee of
service at all.
Because if they do that, people will start leaving that service, saying "it's slow and glitchy, this other service gets much better ratings on SpeedTest.net and all of my friends on this other service say it's more reliable for them", when the reason for that is "this other service" actually has an enforces a ToS against abusive use of the network.
Does anybody get a "guarantee of service" to the public roads? Then why should anyone complain if someone else decides to run a private parade on those public roads, blocking everyone else? Or should we all get a "guarantee of service", and then deny anyone the right to have a parade ever?
Sometimes, you need to share resources without precise, hard and fast constraints, but instead based on social conventions, definitions of completely egregious behavior, and consequences for that completely egregious behavior.