Neighborhood bodegas, which can look like small shops selling food, are a very different type of establishment, and sound like what you’re describing. Perhaps you are ending up in those? (They’re rife in tourist areas!)

I live in the US currently, and can't think of a single supermarket that doesn't have a large fresh fruits and vegetables section, and all of the vegetables you mention are commonly available.

I suspect you were in a big city and going to the equivalent of corner stores (as someone else mentioned, what they call bodegas in NYC at least.

I think you fell into the common traveler's trap of thinking that your lifetime of knowledge of your home country would easily translate to a different country. When you don't know what's available or what to look for, it can indeed be difficult to find things.