As with many FOSS projects, there's 1001 potential todo items to improve the product and not enough resources to get all of them done at once.
Now the resource issue is in much better shape now with the high number of Patreon contributors and the many volunteer devs, but we are not like Unity Tech. who can afford to throw as many developers as needed at an issue or at a request made by users (billions of dollars in revenue will allow that).
If enough people using Godot want vertex painting, the chance of someone contributing patches wanting the same thing will go up (and many patches do involve things the dev. desires to have as his game is worked on).