Because nothing lasts forever-but we have the technology to make it last for a very long time.
Slides and photos do last a while but for them to last for many generations you would have to care for them in a very precise way.
Museums and governments spend hundreds of thousands of dollars to keep original material like film, photos and documents preserved and that means keeping them all in a very tightly controlled environment...they also make digital copies of these films, photos and documents because they know it is the best way of keeping them safe.
How would you feel if you went to look at your photos or slides and found them all faded. All those memories lost forever! And it could happen because of the nature of these photos and slides which are basically chemically based and chemicals break down, and this can happen very quickly. And video tapes are also very prone to deterioration very quickly, after ten years they start down the road of irreparable damage-and once these tapes are damaged there is no way of being able to restore them, so waiting until you have the time to organize these things could be costly.
You may see mold on your tapes...but don't despair, these can often be cleaned for a fairly low cost even if it looks as though they are covered in white mold it is most of the time only on the surface.