It's because a lot of preppers are sovereign citizen, conspiracy theory, right wing nut bags. The idea of prepping got really popular right before the turn of the century and again in the run up to 2012. Obviously nothing happened and many of those people prepping for the end of the Mayan calendar or the 2000 apocalypse looked like certified lunatics. Definitely where a lot of the ridicule and stereotypes come from.
In reality, prepping takes many forms and covers a broad spectrum of ideologies. Lately, prepping has been rebranded, especially in leftist circles, as sustainable living or homesteading. In the general public, you'll probably still get a lot of assumptions that a prepper is a Nazi with a bomb shelter and a million guns. And there are quite a few of those people, to be fair.
In practice, prepping should be more like community resilience, sharing resources, and making as much use as possible of any amount of space you have. Loading up your basement with ten years of MREs and cans of bullets is basically useless. A real prepper stores enough to get through the worst of an emergency but has a plan for continued life in the event the economy completely collapses or something like that.
For example, my wife and I are members of a local farm co-op and have hydroponic gardens in half the corners in almost every room of our house. We have chickens, and our entire front plot of yard is loaded right now with squash, peppers, tomatoes, and more. We give away or trade what we can't consume ourselves. And believe it or not we live just outside of the center of a small town and have neighbors all alongside and behind. I also own guns, and keep a store of a few months worth of shelf stable goods and have a solid selection of tools. It's not set up right now, but I have solar power if I need it and the ability to get rain water collection going.
I don't refer to myself as a prepper because that's not the primary goal of what we are doing, it just so happens to be a bonus of the marriage between community engagement and self-reliance.