The weight imbalance causes them to fall off the plate and usually flip food to the floor. Is there any history to this or a valid reason?
Tableware is made for specific purposes. "Tea" spoons were made specifically for tea; the business end holds just the right amount of sugar for a cup of tea, and the size is just right to stir it. "Table" spoons were made for eating food at the table. They do not have the problem you describe, and people prefer not to use them because the balance is not what they are accustomed to, and a tablespoonful of food is too much to mouth in one gulp.
So the answer to your question is "Because people like them that way."