If you include GPL3 code, your code also effectively also becomes GPL3 licensed. This isn't a problem for companies producing GPL3 software, of course, but most companies keep their source code to themselves.

True, the web loophole is a way around this, assuming the GPL'd code isn't part of the compiled/minified/integrated frontend Javascript libraries that gets sent to the client (because then you have the exact same problem).

It's not that commercial use is prohibited by GPL3, it's more that most businesses do not want to risk accidentally releasing a version of their software that requires them to share all source code to that version. And even if you're producing open source software, GPL3 might still be a problem because of incompatible licenses (see ZFS for an example) if you don't own _all_ the copyright so you can dual-license your software.