Every company takes inputs (labor and physical goods) and produces outputs (products and services). GM is not the only producer of cars, this is a very competitive industry. If they sell a car for $25,000 they can't simply raise the price without consequence - there are competitors' cars for sale. So how will they cover the extra expense demanded by labor? It sounds like you believe executives are paid too much, so the extra money to pay labor should come out of executive compensation.

However, the vast majority of executive compensation is in the form of equity, and labor's demand for more cash will punish the stock price. Plus there really isn't that many executives and you can't squeeze them enough to cover the extra spend on labor.