Mysteries of grocery shopping in Cusco

I recently returned from a 3-week trip to Cusco, Peru to help out my fellow Project Associate who is based there. I live in Lima, so the trip to Cusco, a sizeable city in its own right but quite distinct from Lima's hustle and bustle, was a welcome respite. I most certainly enjoyed the sunny weather and the lack of noise pollution. However, being based in Lima, I have grown accustomed to the everyday conveniences that are typical of a big city. I was reminded of this point during a trip to the grocery store while I was in Cusco, albeit in a very peculiar way.

I was doing some last-minute grocery shopping for dinner one night, wading through the after-work crowds to weigh the head of garlic and the onion that I needed to buy. Upon arriving at the front of the line, the employee in charge of weighing produce informed me that I couldn't buy just one head of garlic or just one onion. Huh? I had a hunch about this considering I had to pluck one of each out of the bags of 6-10 that they were bundled in. Nonetheless, couldn't she just weigh them and charge me the appropriate price based on their weights? The whole situation struck me as particularly odd considering I could walk to the pharmacy on the other side of the store and buy a single packet of shampoo or a single hair tie (I don't use these, but I've seen it done) with no difficulties. The question that lingers in my mind is, if I can buy these other everyday items on a per-unit basis, why not food? I guess I'll have to wait for my next trip outside of Lima to dig deeper into this mystery...