Because than people won’t come to your store. People buy where they can purchase the maximum of their shopping cart in a single place.

That is why you have loss leader grocers, where they pull people with dramatic discounts on specific items, but the total cart costs the same

That's not how it used to work. That's still not how it works in my country. I buy my bread from a specialized shop, my cheese from another, and my fresh produce from yet another. I know people who only buy their meat from a butcher (I do it sometimes, but not always).

It really depends on the countries culture

I understand your point of view. But in cities of all sizes, it's easier to not have to do that. For example in NYC, a medium size city, you can easily go do your shopping in multiple places, and not at the same time.

Yes, and some people do that.

Some consumers go to specific stores to purchase specific qualities of brands.

But most do not, especially for convenience products. You get it where you can.