One is that TLS is married to DNS, and no third-party CA would issue a certificate for localhost. TlS with self-signed certs generated on the spot becomes easy to forge.
Another is that no web browser suppoorts Unix domain sockets or a Windows equivalent as a an access protocol. You are stuck with IP-based access, wee above.
Third is that access to the OS is limited or forbidden inside the browser, for a good reason. Want to run a locally installed compiler from within your web IDE? Tough luck.
Electron solves all of these problems. It doesn't do it in the best way imaginable, but in some cases just a web site cannot be a replacement.
Developers don't use electron because it'll get its own browser engine instance.
If you can't, it's not the browser's fault. I can definitely screen share in Jitsi from the browser, so there's no technical reason Slack shouldn't be able to do the same thing.
I also see this project but you have to write Go to talk to the OS, not Node: https://github.com/zserge/lorca
But if you want broad cross-platform support "just get the locally installed browser engine" suddenly becomes a hard problem, unless this becomes a popular OS feature.
At the same time, companies were locking down WScript due to scripting viruses and HTAs that were impersonating other applications with their configrable icons.