Honestly, the reasons could mostly be summed up as "we already used WordPress for the blog, we now need to add a shop onto the existing site, might as well extend the former rather than rebuild or maintain two systems".

Alternatively, folks that aren't particularly technical hacking together a solution that works 'well enough' for what they need, and agencies that only have the interest/capacity to use one platform deciding that each and every customer needs to use that platform.

If you're a skilled programmer WordPress is probably not a good solution for your issues, but if you're more of a designer or hobbyist that wants a somewhat hacky extendable blog system it's pretty easy to use.

I suspect you missed the joke: A web shell is a script to execute arbitrary commands, i.e. Remote Code Execution.

You may be missing the point that the WP ecosystem is aimed at non-programmers.