They don't compete with each other.
Without net neutrality, they can prioritize, slow down, or block any site or content they want to.
Many people cannot simply switch away.
They have only one ISP to choose from.
You can switch away from Facebook whenever you wish.
https://www.cnet.com/news/fcc-formally-rules-comcasts-thrott...
That's effectively what lead to the situation we are in now. FCC said it's illegal, appeal ruled FCC doesn't have the authority to regulate at the time, FCC makes attempts at making "net neutrality" a thing they can control, and now we are on the verge of having them roll it back.
Basically, ISPs starting pushing the boundaries and a war directly against the concept of net neutrality and that fight is the one we are currently talking about right now, this is just the latest battle.
So it's not a case of "It only came into law 3 years ago!" but rather "We put it back into law after being considered law for 30 years because of a court case that demanded clarification".
Those prior decades were the infancy of the internet. When children go 16 years without wrecking a car, it doesn’t mean they are perfect drivers.