Another perspective: Airlines hire future Captains, and Captains are expected to instruct. Not only do you have to instruct the FO, but you have to get along and interact with many people and act as a leader. Not everyone is "natural" with this or "wants to do it" as in they just can never get enough "interacting-with-people" everyday, but the smart ones realize that these are important skills and that they can overcome the barriers to be successful. If you think you can not overcome these barriers, as has been said above, you might want to be thinking about a different career, because this one requires a lot of the things that you will learn and experience as a flight instructor.

And no, it's not the only way to get "in", but it is one of the main ways to build hours and experience.