I prefer to engage with others as little when I enter a store. I want to find the stuff I want, pay for it, and get out. I'm even willing to spend twice as much time (or more) looking for what I want than I would have if I have go engage with a store assistant.
I'm not exactly socially awkward, either. I just hate shopping. I find stores quite adversarial. I feel that assistants aren't really there to help me; they're there to maximize profits for their employer. I prefer to make a buying decision without the time pressure I feel if I'm being helped. If I'm told "you look great in that" I am unable to figure out if it's really suitable for me, or if the assistant just wants to make a sale, or if the assistant hasn't been able to judge me accurately.
My answer to “Are you after pants or tops today?” will always be “I'm just looking; thanks.”