Ghosts are around because they haven't reincarnated yet. We do not immediately reincarnate upon death simply because we have other things to do, things left undone, a period of contemplation or rest, lots of reasons. Some of us immediately reincarnate upon death and others linger in between as ghosts on this plane or in another plane of existence until they are ready. Rarely do we remember this period although small children seem to be able to remember it vividly sometimes. I don't think we would quite be ready for another life if we reincarnated so quickly although some of us are.  

Really good question though.

Personally,I think a "spirit" of a deceased, needs sometime for R&R before begining over. What I haven't worked out is the time before reincarnation lasts, as there have been observed "ghosts' of persons that have died several hundreds or more years ago.

I've heard theories too of how the way in which someone passes effects their afterlife choices... ie. if it is sudden and tragic, that spirit may linger confused or unable/unwilling to "let go" right away. I agree that it would be difficult to start into another life immediately without some "soul time" to decompress.