The big issue is browser support. According to caniuse, IE and Edge simply have zero support while Opera and Safari have only partial compliance. The current Firefox ESR doesn't support it either (behind a flag).
The polyfill incurs a big performance hit and a lot of devs need to support those browsers for several more years.
That's silly, customized built-ins are not an essential feature. They are a nice to have. You can get by without ever using them, which is indeed what most people do. As does the subject of this discuss, the Lightning Web Components doesn't use that feature at all.
https://reactjs.org/docs/web-components.html
The big issue is browser support. According to caniuse, IE and Edge simply have zero support while Opera and Safari have only partial compliance. The current Firefox ESR doesn't support it either (behind a flag).
The polyfill incurs a big performance hit and a lot of devs need to support those browsers for several more years.