The difference is the scale it allows. The amount of writing a human can generate is relatively limited, even if it is bad writing. GPT and its equivalents allow you to turn electricity into spam with minimal human intervention. That allows unprecedented levels of spam of sufficient quality to make it hard to detect without being examined more closely.

Spam is spam whether made by a human or an AI.

Good, valuable content is good whether made by a human or an AI.

Bad or uninteresting content is spam if it’s being pumped out by an AI, but it’s a learning experience/personal expression if it’s made by a human.

The difference is intent.

It is not, and most of the internet is already trash. We're speaking a lot about scale - but there are currently billions of humans and most of what they put online is already trash. It's not going to materially change - it is currently almost impossible to manually (on your own, at least) figure out what is good, and it will be impossible in the future as well.