Because hummingbirds aren’t humans, as I’ve pointed out here before, and one species’ meat is another species’ poison.
We modern humans are large, sedentary primates whose evolution hasn’t prepared us for our current unnatural abundance of calorie-dense foods, including sugars. We eat vastly more sugars than our ancestors did, and we pay the price in obesity, insulin resistance, type 2 diabetes, and other problems. *
Hummingbirds, on the other hand, are tiny, hyperactive creatures with raging metabolisms fueled in large part by naturally concentrated sugar sources (primarily flower nectar). Our smaller northern species need at minimum the caloric equivalent of ~40% of their body weight in sugar every day just to function. Even at our elevated consumption levels, it would take the average American more than six months to eat 40% of his or her body weight in sugar.
All plants manufacture sugars in their tissues, and many use them to bribe animals for pollination services. Sucrose, a naturally occurring sugar that most of us know as white table sugar, is the main sugar found in the nectars of hummingbird-pollinated flowers and so is the most natural ingredient to put in hummingbird feeders. We get our sucrose from the sap of sugar cane and sugar beets, but it’s chemically identical to the sucrose in flower nectar. (Refining sugar isn’t like refining oil; it involves filtering the contaminants out of the plant juices, including some that are dangerous to hummingbirds, and crystallizing the purified sugars.)
* I’ve also said here before that sugar doesn’t cause diabetes, based on assurances by ostensibly credible organizations, but recent research has established a very strong correlation between sugar availability and type 2 diabetes. It appears that Big Sugar took a page from Big Tobacco’s playbook and worked tirelessly for decades to keep the public from learning the facts about their product.