Because it jumped from an animal to a human, just like every other virus we’ve ever suffered from. If you’ve ever worked in agricultural, you’ll know the massive amount of antibiotics it takes to keep animals raised for food healthy. Take that away, like in a wet market, and it’s not shocking a virus would emerge.
There is also the government response of trying to cover it up instead of coming down hard. If it was from a lab they would have known exactly what it was capable of and that they couldn’t hide it. The lab in question also had a mix of international scientists so it would have been even more difficult to keep a leak under wraps.
There is also the fact that lots of rightwing chuds would love it to have come from a lab, because that shifts the focus from managing a virus, to punishing someone for causing a virus. Turning boring precautions into emotionally satisfying revenge. So people want it to have come from a lab and will jump on to anything that confirms that bias.