The arrangement is, by its very nature, temporary. In this way, contractors are distinguished from payroll employees. Many of us change jobs to break out of this career trap. A few of us become contractors. In the meantime, the range of marketing activity that a company may need frequently exceeds the capacity of what any one of us can effectively do.
Some companies add staff.
Others turn to outside specialists. Small to mid-sized companies, in some cases, never hire marketers. Instead, they break the marketing function into specific core services and periodically contract with different specialists to address those service needs. Either way, the nature of the relationship is temporary. So, as companies become more sophisticated about the particulars they want from marketers, they will tend to outsource those particulars to specialists.