Bad camera quality, not everyone is photogenic , doesn't know how to handle face muscles when taking pics...etc I rather look average in camera but nice irl than the contrary tbh, especially if you are a guy,like who the heck cares if your pic look gorgeous as long as it's decent you are not a girl to give a damn about symmetry and bullshit... That won't get you a long term partner, but your status/financial situation will make it easier for your to get one.

A lot of people don't know how to take good pictures, you see it here all the time, maybe I should write a guide for that. Bad poses, bad expressions, blurry pictures, bad lightning (crucial for a good pic), busy backgrounds, potato quality camera, etc. In real life, your brain can correct for all of these and also sees it like a video instead of a static snapshot at that time. The flip side is if you know how to use the camera properly, you can make it look better than it actually is, there's a subreddit called instagramreality which showcases side by sides of these.

I have this same issue. I'm considered to be reallt attractive in real life but in pictures I often look average. I think it can be due to multiple reasons. For one picture are still images and this makes it so that certain flaws and certain asymmeties that nobody would notice in real life are much more obvious. Also the shape or size of your face or features can kinda change depending on the type of camera or angle as well as distance the picture is taken. It also kinda depends on the lighting or some people are caught in the middle of a weird expression.