"Left-pad was fetched 2,486,696 downloads in just the last month"
There are LOADS of reasons why the current dependence on 3rd party sites & domains of websites is stupid, not just this example.
Security (not serving malware if someone hijacks a domain)
What's wrong with actually storing everything your webpage /site needs on your own server? If a site with a lot of transactions you can even cache whatever it is in RAM, or even on another server.
This is an example of what's wrong with programming and website development. As for people using google's content to make their site go ... Well just stupid and a poke in the eye privacy wise of their users.