Fast releases and automated updates only make it worse, as two minutes after a prefixed property is thought up, a seriously large number of users will have support for it, developers will play with it and then the css code gets stuck on the internet.
By now most developers are in the mindset that it's okay to let a page differ in some browsers as long as the difference is only minor/aesthetic and not a functional handicap. And matching this vendors are perfectly happy with letting developers use prefixed properties as if they were stable.
Since these vendors make up the whatwg and w3c, they need to get their shit together and standardize faster and in the meantime developers need incentives to only use prefixed properties on test sites, which could be as simple as a console message that it should be removed (like how they did with the event.layerx deprecation) or having the user enable test mode in their browser configuration.