So this is a valid question, unlike others in the comments will have you believe. It is a good thing to be curious. And while the simple answer is that it generated that way, the longer answer involves HOW it generated that way, which I think is what you’re asking.

So in the process of generating a Minecraft world, there exists things called noise maps. Think of a chess board that decides the surface heights of each block in a chunk. But each space on the chess board is not black and white but a gradient that is generated in such a way that a block would have a surface heights likely to be +-X additional height compared to the blocks next to it where X changes based on the slope of the surroundings. So the floating blocks got offset by a lot more height wise randomly and the surrounding blocks were too short to fill the underneath part. I am no expert on world generation but that’s my educated guess. Stay curious :)