Because sometime in the future someone will be repairing what you make or what you repaired with modern glues and they will have nothing good to say about you. I said something here. I do a lot of repairs to furniture both old and new and when one comes in glued together with white or yellow or poxy or primate or instant or hot glue gun glue, I have to charge more for the work.
EDIT Fish Glue from Lee Valley is also excellent.
Get some ground hide glue from Joel at Tools for Working Wood or some liquid Old Brown Glue in a bottle and use that for gluing your furniture together or if you are repairing furniture both new and old and everyone will be happy, including the next person that has to deal with broken furniture.
I know I have gone on and on about this, but it is a very important point. If you have used modern glues for repairs or new construction in the past and you change your ways there is redemption. If however you continue to use modern glue to repair old furniture [and some day the furniture you make will be old] then as someone suggested, there is a special place in hell.