Why are some people like this?

I am a male and I have met other people such as the person you described and they only greet those they find valuable for them. These people are selfish and ignorant towards those they find unappealing at times. The fact that they ignore you when you don't wear makeup or don't dress nice is the kind of behavior these people have. I would suggest to ignore these people or at least reply back the way they reply when you don't wear makeup or dress nice, and continue being yourself because in the end, you will eventually find someone who will appreciate who you are despite not wearing makeup nor dressing nice. You sound like a kind person and what you are going through is unreasonable, but is one of the many life challenges people eventually face. It is your choice to be who you are and shouldn't have to change who you are to please others in my opinion. I hope you found my response a little interesting and wish you the best out there.