How do you buy your groceries, do you use a delivery service? I take it you never eat in a restaurant? I'm surprised that you never shop in any brick-and-mortar shops at all. While online shopping is popular, most people still do quite a bit of their shopping in locations where they actually have to meet someone in person at a cash register and hand them a payment.
Terrible analogy. I buy my groceries form a B&M that has a large sign that says groceries. It's sitting there waiting for me 24 hours a day. I know when I go this B&M and hand that person at a cash register that they are not going to rob me.
Buying BTC in person from someone posting on Localbitcoins is more like meeting someone to buy something from Craigslist. You need to agree on a meeting location. They may be late. One of you won't be able to find the meeting spot. They may rob you. They may kill you (highly unlikely but still possible).
It's the job of the localbitcoins advertiser to make sure that the transaction is professional, smooth, easy, and that everyone feels safe. If you find a trustworthy reliable seller (look at their ratings and feedback, pay attention to how they communicate and the arrangements they try to make), it's a lot more like buying something at a garage sale.
Sure, the garage sale wasn't there yesterday, and won't be there tomorrow, but you'll have an address and good directions. The seller/buyer will be there waiting for you when you arrive. The location will be public and safe. You don't go to a garage sale and fear that the seller is going to kill you or rob you. If I'm meeting in a bar or a restaurant, why the heck would someone try and rob me of the $50 they just handed me, when they could get a WHOLE LOT MORE by robing the business we are sitting in? That doesn't even make sense.
