In reflection, based on how the party wizard gets treated in most groups I play in, I have another reason.
Wizards already have to spend both gold and experience to make magic items that benefit the entire party. No reason for the DM to kick them in the beanbag as well. No non-caster classes, by the core rules, is expected to spend experience in the fashion that casters are.
Yes, I know, I know. Have other people spend the experience for the wizard. Sure. In my experience, that really doesn't happen much, since so many of the items that benefit the party are "wizard gear" that the rest of the party has no direct need for and thus doesn't feel like they should be helping with.