Because many forbid firearms in their stores.
Because of the costs.
I live in Texas and many stores do allow guns. I spoke to a friend there today and we’ve both witnessed open carries which is fine by me. The only problem is and I relayed the story to him, one day down the road about a mile away I went into a Valero and I noticed as I walked in this pickup was running and a kid was in a car seat waiting in it. In the store I was behind a guy with an open carry and asked him” Is that your truck running out there (not mentioning the kid), what would you do if someone steals it. He said, “that’s what I got this for.” I thought are you fing crazy, your kids in the truck.
Some sign...Nope!!
Can't claim to have been in every Walmart in Texas, but have been in quite a few, and the only sign I've seen is a prohibition on unlicensed firearms, and that's up at every store that sells alcohol.
Simon Malls prohibit firearms. No idea if that might be a factor.