This isn't necessarily true. The wealthy (who tend to make up the power structure in a capitalist country because of their ability to fund politicians) probably wouldn't mind if working people got rich, so long it met with one condition: JUST AS LONG AS
IT DOESN'T MAKE ME INTO ONE OF THEM.
While technology increases the size of the pie that we all share, the share of the pie taken up by those who own and those who work can fluctuate in one direction or the other, depending on the politics of the day. It's true that even if the workers' share of the pie is getting smaller, their standard of living can still increase, so long as the size of the pie itself is getting larger at a faster rate.
The share of the pie, that is, the ability of a person to pay for what he wants, is what determines how resources (like raw materials and labor) are divided in a market economy. The more the owning class can spend, the greater the percentage of the economy's resources are devoted to producing goods for the wealthy. Thus the lifestyle of the wealthy capitalist class is the result of taking away raw materials and labor that could otherwise be producing goods and services for the workers.
One possible solution is to allow workers to vote on all the salary levels within the company. This would likely decrease the gap in income, while still rewarding those that are considered valuable employees. Market pricing would continue to determine the prices of goods, rewarding companies that produce the best products.
The problem with this solution is that it would mean taking away control of the company from the owners. Since many have spent much of their lives putting these companies together, such a change would be met with great resistance from the class that funds the politicians and controls the media (and thus controls the country).
However, much of the resistance is merely based on pride. It may not be so much the owning class wanting better goods, but they want the recognition for having achieved something or for being able to show off their latest acquisitions or charitable donations to others.
If the political climate of a country is to change, some other way would need to be found to satisfy the sense of pride and accomplishment of those who control the country, while removing their power to redistribute the economic resources of the country with their purchasing power.