Why is it most liberals seem to focus on gun control, and gloss over better mental health treatment, as the most effective solution to this problem? They seem to hardly even mention it.
Because nobody - literally nobody - knows what that would even mean.
At the very least, make it much easier to involuntarily commit people. Basically, roll back the Reagan-era "reforms" on mental health care that made it basically impossible to institutionalize people who hadn't already committed serious crimes. (Those reforms are often blamed on Reagan but were certainly also pushed for by congressional Democrats--there's plenty of blame to go around on 1980s politicians.)