An LLM can be trained to find relevant knowledge online. It doesn’t have to be trained on the entirety of all existing knowledge.
Why do you think chatGPT lost its Web Search plugin lately? Copyright lawsuits. You can't even use copyrighted content in the prompt because it will make the model makers liable.
The would possibly need to apply some effort to appear human, but that should only throttle the rate, not stop their scraping all together.
Clearly, places like Reddit have wised up to this and are making API usages non-free for example, so while it's not impossible, you can see the limitations being put into place already. Twitter is another one.
It seems like all this data is now considered gold and people lock up gold?