Because most photographers will look for defects, they will not really judge the photo they will try to see if they can find mistakes in composition or lighting etc. and also because their ego gets in the way and they will not admit that a photo of another photographer is better than their photos, when you have an ego that does not let you see impartially so how in the hell can you really judge a photo.
If you really want to know if your photos are good or bad, do this;
show them to at least 20 people, these people can not be friends or relatives, they have to be complete strangers, then you will find out if your photos are good or if they suck.
Ask people to grade them from 1 to 10 being 10 the goodest of the most good photo. People that do not care who you are are going to be the best judges, but you have to ask at least 20 people, or you can ask more people, the more the better.
There you have it, so don’t go showing your photos to the people in your photographic club or group or family because your family will say your photos are great because they are your family, and the photographic club members will tell you “Keep it up one of these days you’ll take a decent photo maybe when you have more experience”