It's just that the first user (who might not be any good with Photoshop at all) finds it cool, makes a tutorial about it and another newbie thinks "hey that looks cool I'll try it", and later on makes a tutorial on it yet again. The bad circle is endless. In fact, many signature tutorials are of the same difficulty but maybe a bit different - brushing isn't hard because the brushes themselves are already made by other artists: these type of signatures consists of other artists work, not your own.
It looks easier to just use a brush and hit it with every power you got but other signatures are as easy to make with just a bit of theory and practice. Brushes are the easiest methods to get away with and create a custom signature with minimum effort.
To future beginners: I suggest you skip any tutorial that starts with "find your render, use brush X and color Y and brush the entire signature" and instead go for the other beginner-friendly guides that tell you WHY you should be doing a certain thing in a signature. You can never evolve by relying on other people.