Why are Edison screws allowed to be used?
The Edison lamp base design dates predates all twentieth-century safety regulations (because it predates the twentieth century altogether).  Having light bulbs screw in and out is not great in environments where vibration is a problem, and replacing a bulb which has just burned out while in use may require use of a cloth to hold it, but it is generally advantageous to have a lamp which can be inserted or removed by handling the bulb rather than having to mess with the holder.  Making a metal thread assembly that can mate reliably with a metal socket is easier than trying to mold threads into glass, and having one contact in the center of the socket is easier than trying to have two concentric contacts.

While I doubt the Edison base would be approved by any safety agency if it were being introduced as an entirely new product, it has been effectively "grandfathered in" because it has been used for a long time, people are familiar with it.  A "safer" design that people aren't familiar with might lead to more accidents than the century-old design which, for all its imperfections, is well understood.