Six years is indeed a long time for her to say something like this. As far as her reasons, they can be multiple. 1/ she is not fully emotionally invested in the relationship and thinks that by making these statements, she'd in some way remain distant. The reason for emotional distancing may be something she's fully aware of as in fear of commitment or not really aware of completely as in using this statement as some sort of a defense mechanism (denial) 2/ She may truly believe what she's saying- she wants to keep others rather than you at a distance phrasing it keeping personal from business life (whether everyone else is falling into that business category is another issue) 3/ she may on some level want to keep her options in case there is a heartbreak (but you've already been an item for six years)
Even if it seems to you that there is underlying reason, she may not be conscious of it and the behavior may be driven by subconscious forces such as- surrendering to the relationship she fears she'd lose her sense of self and freedom.