There's all sorts of things you can tell yourself to justify ridding yourself of stuff. Whatever that stuff may be. For example, if I die tomorrow would I want my spouse and/or family to have to deal with my collection or my junk? I had an older brother who died last week and although he wasn't in good health, he passed rather abruptly. He was a hoarder. Now we are faced with dealing with the aftermath as a family. There are some valuables in the mix but they won't benefit him now.
Not to sound maudlin but at some point we all go and last time I checked all we take with us is what God blessed us with when we were born. Along with some titanium screws perhaps, if we weren't so lucky. But you really can't take it with you and you really shouldn't leave it for somebody else to deal with. They have enough problems.
Another thing is the fact you want another frame and selling off the bits and bobs will fund it. That's a win/win, downsizing is good. Going from multiple bikes down to one really cool one is a good trade.
What could you do with the space that results from eliminating some bikes and parts? Make room for something else that is more useful to you at this point in your life?
Lots of reasons, you just have to look for them.