number one reason is that people don't usually openly tell a good-looking person in front of them that they think are good-looking. maybe because they do not want that person to become maghroor and start acting haughty. maybe they feel by telling them this that person will get offended or think that they are just trying to flatter them for a purpose.
second reason is beauty is in the eye of the beholder. maybe a girl who one guy thinks is really pretty another guy may think is average.
also sometimes peoples personality, dressing, style of speaking, persona etc influence how someone sees them and on these basis they may find a person more or less attractive