Answer: It's a combination of lack of understanding and conspiracy theorists. Usually both. Some people think 5g causes cancer, which isn't true. Only ionizing radiation causes cancer, and 5g is not ionizing radiation. It doesn't even penetrate human skin beyond a few millimeters. (Note: non ionizing radiation such as microwaves can cause burns at high enough intensity but that's different, and not an issue unless you're fornicating with a 5G router.)

There's also people that think Huawei, a Chinese company that is manufacturing 5g modems is going to steal our data and send it to the Chinese government. The U.S and other countries are considering banning their products entirely, the U.S is planning on enacting a ban on may 15th, so it's unlikely they will be an issue.

I think that's the gist of the concerns that sounds somewhat reasonable. The rest I've seen are pretty out there conspiracy theories.