I don't have a good in game reason. I just leave it as unexplained.
It does have a metagame reason. It helps control the magic level of the game. The first campain we started with 3ed our wizard made lots of items but fell almost 3 levels behind. The lost of levels was only partly made up by the power gained by the items. The next wizard player used a different approach. He rationed his expenitures to 10% of gained XP. By doing this the party had the items it needed and/or wanted while the wizard advancement felt more like 1st or 2nd edition.
There are others ways of controling the level of magic items that others have used. Maybe some one will post about one that works.
(edit to correct word)
It does have a metagame reason. It helps control the magic level of the game. The first campain we started with 3ed our wizard made lots of items but fell almost 3 levels behind. The lost of levels was only partly made up by the power gained by the items. The next wizard player used a different approach. He rationed his expenitures to 10% of gained XP. By doing this the party had the items it needed and/or wanted while the wizard advancement felt more like 1st or 2nd edition.
There are others ways of controling the level of magic items that others have used. Maybe some one will post about one that works.
(edit to correct word)
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