Because they believe if they don't blow their own horns no one else will.
Modesty shows confidence and self-esteem better than any braggadocio can. It is a TELL of insecurity. Do I know that for a fact? No. But it seems very logical to me that's what it represents. If you are really good you don't have to brag about it. People know. Even so why would it matter? Isn't the most important thing what YOU think of YOU? Shouldn't it be?
