Article title is a bit click-baity. The article is only talking about Docker Desktop on Mac or Windows. Docker Linux have not been using microVMs for a decade.
Neither XNU/Darwin nor Windows have had an oob option for running Linux images. WSL is rather new in comparison.
Developers, ime, also don't really want to start building their own container images for auxiliary services like, e.g., PostgreSQL, MongoDB or MySQL.
Having virtualized Linux support means you can take the existing ecosystem and make it accessible to the other two. It also means docker itself doesn't have to fully embrace & support either which is part of why it's "cheaper".
You sure could. Kata containers have been around for a long time; I remember putting them into production back in 2017 or so (and dealing with filesystem forwarding issues). https://lwn.net/Articles/755230/