Adding solder mask is an optional step. If you use an etch resist like toner or dry film, you have a functional PCB even without solder mask. The solder mask is the unnecessary step that can be skipped.

Using solder mask as an etch resist, you would have to scrape away the remaining mask in areas where you want exposed pads - you would either need another mask for this, or to manually scrape each pad, adding an unnecessary step.

You'd still need to apply a second layer after the etching. There's less risk of misalignment if the solder mask is all applied at once, so might as well use the more appropriate resist.

Here's a video where one guy already has done this successfully, even with diy silkscreen https://youtu.be/gSjTScsCXRM