maybe make it illegal to keep your gun in your car?
Unfortunately this is not possible. There are many businesses and such that have signs to the effect of “no guns in here.” In some states those signs hold no legal weight, but in some they do. In states where they do hold legal weight, your choice becomes
Just never carry because the grocery store I’ll be in for 15 minutes out of my day has a sign they think will keep mass murderers out (spoiler warning: mass murderers target those signs. Not that it’s more likely you’ll get shot there necessarily, just that their signs only matter to people who aren’t about to murder 25 people, as the murderer has other crimes to worry about vs me, where I just want some damn nuggies so prison actually matters to me.)
Carry in the store illegally. Honestly more people are doing this than you think, but as I said in some states this can become an issue for you.
Leave it in the car while I’m in the grocery store. Legal, not exactly safe, but since I am literally legally forced to be unsafe: “not my fault.” If you want to charge people with leaving a gun in a locked car and then the gun gets stolen, you have to at least meet halfway and let people with a permit carry at all times and not force them to leave it in the car. You may say "just go to a competing business. Well the way my state law is set up you can’t carry in ANY bank regardless of permission, any government building, and a few more places. And I’m fine with either, make them leave it and no charge or let them carry it and charge if they don’t, but I’m not fine with “you have to leave that in the car even though you’d rather leave it in the holster, and if it gets stolen we’ll put you in prison for life.”