Because they did disappear overnight. Microsoft changed their licensing so that screens had to be under 10.1" to get reduced pricing with Windows 7. By this time, netbooks had already creeped into the 11" size range.

The second thing that caused them to die out was increased memory requirements. The chipset used in the majority of netbooks topped out at a single 2GB dimm. With bloated apps like Chrome and Electron, the systems became practically unusable.

There are various Chromebooks nearly as small, with low weight.

I've installed GalliumOS on mine, and it is working pretty good. Light and handy, though battery life isn't quite as good as it was with ChromeOS.

The main thing missing is Ethernet, but I have a dongle for that.

Microsoft and Intel happened to them.

For example: https://www.pcworld.com/article/169919/5_netbooks_microsoft_...

Thin margins and the realization that if netbooks became popular most people would likely not buy normal laptops. Why make the one thing that kills the rest of your product line?

Smartphones with 6" screens I would imagine.