Why bother to steal free to read novels?
Because the more novels they get their hands on, the more people will read them and the more visibility their site will have. Tbh, it's extremely hard to stop an aggregator site from stealing your stuff unless your novel is licensed somewhere. In that case, sending legal documents can solve the problem. Yet even then, most of them won't take you seriously because they do not expect you to go that far.
Suing the owner of an unknown aggregator site in a random country is as successful as trying to kill a fly with chopsticks. That is the reason why they feel so confident.
The only few cases where I've seen aggregators being in a dire situation was when they got their ass kicked by a major publishing company. Still, it's only 1 site and there are thousands of them.
So imo you should just do as if they do not exist, and try to bring your readers into the sites you are using to post your story. You should also create a discord. It won't solve the issue, but at least your readers will form a community and read your chapters on the sites you are using to post them, not on a random site.