Because some people wrongly assume that any coins which has not moved in 5 or 10 years is "lost" and therefore removed from the supply, and therefore any coins which have been dormant for this long and then "wake up" again are effectively inflating the current supply. This is obviously nonsense, as has been proved many many times in the past. The only coins which can be removed from the supply are those which are provably lost or burnt, and those coins number a few thousand at most.
Even if no coins are lost, still the bitcoin supply is very less and limited and if the demand grows, the price will rise significantly. However, there will be constant loss of bitcoins as people might forget their passphrases and some bitcoin holders might die too without telling their keys to anyone, and a lot of other reasons.