Maybe because this method is not that popular in the American sharpening community. This is firstly, and secondly, it is a little "dirty", requires compliance with "hygiene" when changing powders, and it is even better to use different sheets of glass for different powders. It's inconvenient.
Is he better? Yes, in my opinion, and not only in my opinion. First, because when leveling or flatting, there is no flexible backing on the glass, as in the case of using sandpaper. If you use stones for flatting, then I have not met a perfectly flat stone, as a rule, they all have a hump in the middle. And then, as a result of grinding, a pit is formed on the whetstone. The glass must be thick enough to eliminate its deflections when the stone is proofed.
Secondly, the area of the glass is several times larger than the area of any flatting stone, which makes it possible to use different movements of the stone during alignment or proofing and thus avoid the formation of pits, humps or the formation of a propeller-shaped surface of the stone.
The difference is especially clear on hard natural or synthetic stones
Is it all to use and is it worth bothering with? Each user's case. As I wrote above, here as in many other fields, the Pareto rule works: 20% of work - 80% of the result, the next 80% of work - only 20% of the result