Because there comes a point in some basses 'life' when it means more to the owner than just being a musical instrument.
My '73 Rickenbacker may not be what you would neccesarily call 'vintage' but out of the 50 or so years that bass guitars have been around it has seen 35 of them - so it's getting there. I got it in 1976 after saving hard out of saturday job wages and my parents putting up half the money for my 21st birthday. Since then it's seen me through many different bands of hugely diverse genres. It was my first 'top end' bass (back then there were Fenders, Gibsons, Ricks and not much else that would pass for 'pro' grade instruments) and it helped shape me as a bassist. It helped me earn enough gig money to make the difference between my family eating and not eating and survived being sold itself any number of times to pay a mortgage instalment or a couple of weeks groceries (Money was actually changing hands one time when my wife said "don't sell it - we'll get through somehow"). For 20 odd years it was my only bass and even when I had reached an age when solvency was no longer an issue it was still helping pay college fees.
In the last 10 years I have been able to indulge myself with a couple of extra 'top end' basses and a couple of cheapy spare basses, but it's only in the last 4 years that my 5 string fretless has been my main bass and taken over from the Rick.
Of course the Rick is meant to be played and it still does a fair number of gigs (just not every gig like it used to). It's also used for recording and sounds great.
But, one of the reasons that it is in semi retirement is that it has been such a large part in the fabric of my life that I really couldn't bear to see it damaged or stolen. I will never sell it, but what I am going to do is search out for a young bassist (I already have one in mund) who is struggling for a decent bass and when I have assured myself that they have the talent and lifelong passion, and will cherish my Rick as much as I do, I'm going to give it to them.