Because I've always done this, right from when I was at secondary school and it just seems normal to me. However I've read various MN threads recently where people have said they find people who cut others off weird.
I very rarely fall out with anyone, I just sort of disengage if I realise that they're not a nice person or if they do something unkind towards me. I might say a brief 'hello' if I see them, but a couple of people have pissed me off so much that I now just walk past them and totally blank them. One of these is a mum at my DCs school; her daughter was regularly unpleasant to my DD a couple of years ago, and the mum tried to turn the tables and blame my DD, and sent me several nasty texts. I decided not to even bother speaking to her anymore, and now just walk past her.
To add, I have lots of lovely friends, and am always making new acquaintances, and generally do 'get on' with people. I think I just don't/won't tolerate people acting in a shitty manner towards me.