Why would your company want to pay you more than they are right now? What is their incentive? Retention?
Other companies are willing to pay you more because they need your expertise and you are offering a higher level of skill/experience than you offered the company you are with now.
Retention is a very good reason to do so; replacing someone good is always hard (and expensive) in terms of hiring and ramp-up times. If you don't adjust your employees' salaries to reduce the incentive to job-hop in order to get a raise, you're basically asking your best employees to leave.
Other companies are willing to pay you more because they need your expertise and you are offering a higher level of skill/experience than you offered the company you are with now.