The same thing as with tooooooons of things: scale.

Nobody cares if one dude steals office supplies at work. Now, if everyone stats doing it, or if the single guy steals everything, then action is taken.

Nobody cares if a random person draws in the same style and with same characters as you, but if they start to sell them, or god forbid, out-sell you, then there is a problem.

Nobody cares (except police I guess) if a random driver drives double the speed limit and annoys people living next to the road on the weekends, but when tons of people do it, you get speed bumps.

Nobody cares if few people pirate movies, but when it gets to mainstream and companies notice that there might be money being lost. Then you get whatever we have now.

Nobody cares if the mudhill behind your house erodes a bit and you get mud on your shoes. Have a bunch of that erode and you realise the danger...

You have been fine-tuning your own writing style for a decade and random schmuck starts to write similarly, you probably don't care. No harm done. Now, get an AI to write 10 000 books in a weekend and someone starts to sell them... well now you have a completely different problem.

On a fundamental level the exact same thing is happening, yet action is only taken after a certain threshold is step over.