As someone who advocates that those who wish to improve their selling results use more “I” and “me” and less “we” and “us,” I'd like to suggest another reason for the phenomenon.
I think your credibility is less when you use “we” and “us.”
Think about it: you ask a question of a salesperson, and they say, “We stand behind our product,” what does that mean? But when she says, “I stand behind this product,” well, then, you know whom you’ll be calling if there’s ever a problem!
It’s a skeptical world.
There’s an especially large amount of skepticism in play when people are being sold.
One way to get past that is to indicate that YOU are the person with whom the buyer will be dealing if there is a problem. That YOU are willing to take their phone call or email, and that YOU will find the resources to clarify anything that’s unclear.
The underlying issue, here, is TRUST. We don’t trust many people these days – especially not salespeople – but when we know there’s a person to whom we can turn if the purchase creates a problem, we may be more likely to choose to buy.