I used to have a few of my own washes, and filters... just Windsor Newton oil paint and some mineral spirit in a bottle, it worked well enough but seemed to break down over time. It was cheap enough, I'd just dump it and put some fresh oil and spirit and use it for a while but my mixes weren't always consistent.

Having them pre made seems a lot less work, every time you open it it's the same and the bottle last way longer than my home brewed stuff used to.