Because they’re tiny. Some are invisible without magnification. Even the bigger ones are smaller than a grain of rice, and an unusually small grain at that.

All it takes is walking through dirt that’s been exposed to something infected. It doesn’t even have to be where an animal poops directly on the spot, some eggs can handle being washed away during rain to somewhere else. Which means that even pavement isn’t completely risk free where avoiding bare ground would help.

You ever watch crime shows where they bring up Locard’s principle? The idea is that no matter where you go or what you do, you will transfer something from one place to the next. That something may be indistinguishable from the environment, but we’re swimming through clouds of dust and microbes every step we take.

Every step you take, barefoot or shod, you’re in contact with something. Teeny tiny pieces will be picked up. It may fall off the very next step, or a dozen later, but you’re carrying things along, even on the slickest, smoothest shoes. There’s little microscopic textures that grab things.

An egg for a parasite is usually going to be great at sticking to things. That’s how they find new hosts. Some of them can survive for scary amounts of time in fairly difficult conditions.

The question isn’t whether or not you’ve ever carried something like that into your house, it’s how many and how often.