Because we grew crops historically for that in mind. About 5,000 years ago, if you wanted a watermelon, you'd get something about the size of a strawberry that would taste bitter as hell. Watermelons today have about 4-6 times the sugar content as they did thousands of years ago. They taste better because we specifically grew and cultivated them to do just that. Oh, and guess what? During that time period... it also increased in size by multitudes WHILE improving flavor.