Why? Because photographers want the gardens to look lush and full...which means the gardener must put off harvesting her crops until after the photo shoot. And putting off harvesting is the #1 Biggest Mistake you can make when growing vegetables.
Last year, I explained that failure to harvest herbs often leads to declining harvests. The same is true with vegetables. Often, gardeners get a few peas or cucumbers on, for example, and think, "I'll wait to harvest those until I have enough to serve for lunch." Or enough to freeze. Or to can. But when vegetables sit on the plant, ripe and ready to harvest, the plant thinks it should start going to seed - so it stops producing food and focuses its energy on making seeds. This naturally results in small crops and plants that go to seed early. (Besides, vegetables that sit on the plant for very long quickly loose their flavor and nutrition.)
But the fix is easy: Pick veggies as soon as they are ripe! Instead of letting those peas or cucumbers stay on the vine, pick them. (Eat 'em fresh, or add them to a salad.) If you do this, you will get the most from your plants.