Taking all the flexibility away would be fantastic. I wish all webapps looked as close as possible to native apps, and couldn't style things like buttons.
This is a meaningless statement, because even native apps don't "look like" native apps. No operating system has a universal style across all native apps, and some (Windows) aren't even internally consistent with the OS's apps.
Imo, an app that isn’t using Cocoa (or carbon for older apps) widgets on macOS isn’t “native”. If you just use the default widget set on macOS, your app will have a consistent style with every other native app. If you use Qt, Swing, Electron, etc., your app will look out of place and break in ways that annoy at least some of your users.
I get your point, though; I'm a Linux desktop user, and have to contend with apps written using a variety of UI framework. For that reason, I tend to try to stick with GTK apps, as I prefer consistency. I have Qt set up to use/emulate GTK's theming as close as possible. No, it's not perfect, but most things on my desktop look pretty consistent.
Except webapps, of course! They break everything when it comes to consistency. I wish that wasn't the case.