Google indexes other people's content. All Google has to say is, "Sorry, we're not in control of the content others make, our automated systems follow an algorithm we're unable to make one-off tweaks to." It could conceivably cost Google $1MM to make a one-off tweak to their algorithm in terms of programming and testing time.

Wikipedia on the other hand is all content. They have no plausible response other than, "Yeah, it would take 5 minutes to update that but we won't do that for you." Hell, all they'd really have to do is let the advertiser update it as they want and then instruct editors to do nothing.

It really is just different for this and a number of other reasons.