I can think of a lot of reasons, people's minds work differently. As I said in an earlier post, the desire to "disappear completely" leads people into natural or remote places to suicide every year. It's a symbol or metaphor for them wishing they could undo what they have done in their life.
Most of us have done something we wish we could undo. For most of us, it's minor - we broke something precious to ourselves or to someone else. But the desire to undo or unmake is part of rite and ritual for humans all over the planet. Grief alone makes many persons wish to be gone from this plane and to be invisible.
Then, in this case, there's the additional possibility that either BL simply doesn't understand how much harder it would be for Gabby's family. and for his own family if he couldn't be found. I do not think BL had a strong theory of other minds. I think he was very much inside his own head, had lots of intricate fantasies.
There are other possibilities as well. Of the many people who go missing every year, many are not found and do not want to be found. I believe that one of my. childhood friends committed suicide and did not want to be found. It's been 35 years and he's still listed in the Charlie Project. He's out in the mountains near where I live - somewhere.