Why don't some people have webbing under their tongue?

The presence or absence of webbing under the tongue is primarily determined by genetics. Some individuals may have a condition known as ankyloglossia, or tongue-tie, which involves a shorter or thicker frenulum, resulting in a webbed appearance. Others may have a normal frenulum, which allows for more freedom of movement of the tongue. Variations in frenulum development are simply part of human anatomical diversity.