Probably because if the NPCs just said "Hello", or "Morning", there'd be a fresh round of whinging, where all the whingers whinge about "NPCs only ever say hello to me, they don't even recognise my awesome Dragonbone armour Bethesda you svck and my game is ruined ragespasm spasm spasm RAWR!". Not to mention the ubiquitous "Bethesda, my immersion is ROONED!" No matter what they do, they can't win. There's the reality for you.
There will always be some percentage of people complaining. But don't you think
(a figure of speech. You obviously don't) that when people complain about a single issue repeatedly, there might really
be an issue? [censored], the "arrow in the knee" line being so repetitive might be one of the most recognized things about this game in any media. TES games seem to be all about wish-fulfillment with instant gratification now, so I'd personally think that all the other back-patting, after not doing much of anything, would be enough to keep the people in need of constant praise happy.
Like I keep posting, if anyone feels that strongly about the matter, rather than sit there and curse the darkness, light a candle! http://jobs.zenimax.com/
Can't speak for anyone else, but I've worked in the software industry, and have no desire to go back. It wasn't the customers however who made me leave the industry. "Why don't you do it better yourself!" is red herring anyway. Most people don't have the skills to, say, bake and decorate a wedding cake, but they do recognize when someone's used motor oil in the recipe. Customers have the right to complain, give feedback and criticize the product. It's the company's business whether they'll listen.