Because your choice of phone brand does not tie you to a specific grocery where you can buy rice crispies. You can simply cross the street and shop at a competing grocery.
If I go to a different grocery store, I can get products that are compatible with everything else I own.
If we go with the store=phone analogy, then somehow my brand A flour and my brand B milk are magnetically repulsed and I can't make pancakes. It's not good for consumers.