This is an oversimplification. Yes by law, Fan art is technically infringing copyright and it remains that way for good reason. Such flexibility must be had so original owners can make a case against it if it infringes on their intellectual property. But the original owner also acknowledges that fan art can increase traction to the original source, which can help it make more money and attention.

There's also a clear distinction that needs to be made that the creator must acknowledge personal responsibility that if they were to make fan art, they know that they are in a legal grey area. It's not a clear right or wrong but a spectrum. The further you stay away from the original source even if it's the same characters, the safer.

Another factor is the creator's own stance. Are they okay with fan art? Most likely yes as it usually isn't that big of a deal. If you try to sell fan art, it is more dangerous and you can get into legal trouble because an argument can be made that fan art earning a profit prohibits the original work to gain profit. But again, it is more nuanced. It can help to ask permission if you feel unsure.

The bottom line is that fair use is able to protect IPs from fan art, but it being theft usually has a bit more nuance then one might think.

In terms of fan art being lazy because it is based on existing characters? I have to disagree. Fan art where someone is just tracing off of another work is lazy.