Ground isn’t stable. A house by contrast weighs 100,000-200,000 pounds. The dirt under the house will not only settle under the weight, but will continually shift due to surface and underground water movement. To prevent that, you need grading to make the land flat and direct water away from the house. You need to tamp down the soil so it doesn’t compress. You need drainage layers to allow water to percolate downwards (instead of saturating the dirt right under the house). You need layers to keep plants from growing straight through the middle of the floor.

Even a simple concrete pad will crack and heave in a few years without a proper foundation. We recently had a 8x30 paver patio/driveway built in front of our house. It took a team of 4-5 guys more than a week just to dig out the foundation, lay gravel drainage layers and landscape sheeting. Actually putting down the pavers took just a couple of days. And that’s for a patio that’s supporting at most 10,000 pounds of cars, not 200,000 pounds of house.

Typically the alternative is a concrete pad. That's what my brother's fairly large house on the coast of Maine has.

I assume if you just put in posts those would tend to rot over time.

ADDED: Per sibling comment, a concrete pad is often not about just pouring a load of concrete.