Because they come from a business background, executives tend to have little to no understanding of their company's product or service — whether it's frozen potatoes or deep tissue massages. why, therefore, would an executive ever bother to invest in his company's product or service when he can invest in sales, marketing, account management, or something else that has meaning in his world? what is most clearly the central idea of the paragraph?

The central idea of the paragraph is that the company tends to focus on their product or service despite without or little understanding of it. So, the writer insists that, instead of focusing on the product, divert the attention to other areas of business. 

In getting the central idea of a paragraph, determine first the topic of the paragraph. Once you have identified it, it is easy to know the main idea. The writer can put the main idea in the first, middle, or last sentence. Mostly, writers put it in the first sentence. 

My motto has always been ''live life to the fullest.''