What's the point of pre-planning your exit?

It seems that planning and anticipating a timely, successful exit is part of developing a startup for many founders. Even some investors and VCs specifically ask "What is your exit strategy?".

Coming from a pure Computer Science background, I don't see the point in that. Perhaps I am naive or old-fashioned, but shouldn't everyone strive to build a company or product that lasts as long as possible and stay involved for as long as possible? Pre-planning an exit feels like starting off a relationship and not planning for marriage, but for breakup instead.

I suppose everyone would get involved in a product they spent years building, developing and marketing. Seeing it grow from a simple idea to sustainable business, so how can you give all that away and see someone else perhaps driving it into a direction you never hoped for?