The secret is near the metal ferrule.  If paint gets packed up too close to the ferrule, it is almost impossible to get out.  The oil can be removed by using soaps and solvents – but the pigment gets locked in.  Eventually, on finer hair brushes such as sables, the pigment cuts through the hair, giving the brush a splayed effect. Terrible for painting.
The solution is to keep the paint half way back from the tip.  That way, the pigment doesn’t get jammed tight in the hairs by the ferrule. It is possible to remove most of the pigment when cleaning, and the bristles will last much longer.  The brush in the photo looks almost usable.  However, there is a solid core of pigment inside the bristles that you can’t see.  It reveals itself in the green coloring by the ferrule. This brush is about as usable as a strip of cardboard!