Truth be told the whole front-end ecosystem relies on those servers being available, so in their absence any development everywhere would grind to a halt - as it almost did during the left-pad scandal.

While I agree that this is not exactly the sanest approach, within the ecosystem there's no incentive to work differently.

Also, like someone else mentioned - dependency hell only got worse over time - setting up a new project you're likely to have several versions of the same library in your node_modules.