It’s very clearly a mental health and existing-law enforcement crisis, and not one that the political parties are interested in actually solving, because it’s useful to them.
Owning a gun in the US is a political statement about the second amendment, which while laudable on paper is clearly a failure since none of the collective gun ownership actually does anything to keep the government/law enforcement honest (I'd argue it makes it worse as every cop is more on the edge that they would be otherwise).
This leads many people to obtain guns that otherwise have no time/desire to learn & practice responsible firearm ownership and enjoy their firearm safely without harming anyone. The firearm then mostly collects dust until it gets misused in a freak accident or falls into the wrong hands.
Guns per 100 people - Finland 2017: 32.49 (total of 1.79 million) [1] - US 2017: 120.5 (total of 393 million) [2]
All gun deaths - Finland 2017: 138 [1] (~77.09 deaths per million guns) - US 2017: 39773 [2] (~101.2 deaths per million guns) <- A bit higher
Mean death rate per million in mass shootings, 2009-2015 [3] - Finland: 0.132 <- A bit higher - US: 0.089
[1] https://www.gunpolicy.org/firearms/region/finland [2] https://www.gunpolicy.org/firearms/region/united-states [3] https://worldpopulationreview.com/country-rankings/mass-shoo...
The data seems to suggest that the number of deaths and mass shootings sort of tracks with the number of guns, and that Finland isn't particularly better off "per gun".
In the US it’s a multi-faceted issue of mental health, lack of effective enforcement of existing laws, acceptance and promotion of gun and gang culture in music and movies, and the political utility of a continuing problem. Democrats in particular need ongoing crisis and a large dependent population to stay in power - this is why they so visibly pander to the social movement of the moment. They need the “gun issue” to stay present and emotional, they have zero solutions on iffer. Crying “Ban assault weapons!” and then being completely unwilling to define it or describe how they’d enforce it effectively just lays bare their cowardice and intellectual weakness.