You have to acquire knowledge to make yourself a valuable resource to your company. The average turnover in the corporate sphere is now two and a half years. Your current employer is unlikely to want to pay you more for doing the same job within the firm. You'll get raises every couple of years, but unless you are being actively promoted, these won't reflect the increases in the market. You're far more likely to earn more money if you go and work for one of your competitors.
Businesses are like people.
They can be narrow-minded and conservative. If you work for a number of companies in say, a ten year period, the chances are you'd learn far more about your industry as a whole than you would working for a single company. The person who keeps moving is also exposed to a number of different operational systems, software and business cultures. Remaining in one job is likely to lead to over-specialization in one area. The people who are advancing fastest in our present economy are the people who realize that they are part of a mobile and professional workforce.