Hi, welcome to your first day on the Internet. Since you're new, let me tell you how things work around here.
There are probably dozens of web sites similar to Wikipedia. But Wikipedia is on the first page of search engine results for just about anything you search for. Why is that? Because people have learned that they can trust them over the last 12.5 years.
When you go to Wikipedia, you know that when you're looking for information on the Battle of Hastings that you aren't going to see ads for anatomy enlargement pills. You won't see any advertising at all in fact. You know that the community at large does a decent job at removing biased information. You know that a company can't buy their way into hiding negative information or promoting positive information.
This level of trust is what causes people to link to Wikipedia thousands of times per day.
So let's say Wikipedia takes your advice. They put a small unobtrusive text advert on each page. Suddenly you're searching for information on acne and an ad for "Acbegone" pops up that promises to cure your problem for 3 easy payments of $19.95. Acbegone ends up becoming a huge advertiser with Wikipedia - spending $1 million per month on advertising. Suddenly Wikipedia gets The Phone Call. "Hi, this is Acbegone. We'd love to continue advertising on your site but your article on acne mentions 10 other products. Get rid of those and we'll double our ad spend with you. Don't get rid of them and we'll be forced to stop advertising." Wikipedia can't make do without the income they've become accustomed to so they make editorial decisions to not mention any product - but still there's that ad from Acbegone. Suddenly Wikipedia seems like one huge cheesey ad. People stop trusting it. People stop linking to it. It stops coming up in search engine results.
For a real world excample, see http://en.wikipedia.org/wiki/Digg#Digg_v4
Look at that - a link to Wikipedia.
Everyone goes to Google.com to search for things. You know that when you do a search, you're going to see helpful related adverts.
That level of conflict of interests and possible abuse, privacy concerns etc, means that the entire world uses google as their search engine.
Oh and they make billions in profit.
Your hypothesis about an advertiser asking wikipedia to alter content surely applies to google search results.
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.
>and then instruct editors to do nothing.
Yeah good luck with getting wikipedia editors to comply with that request!
A site like wikipedia would likely have thousands upon thousands of advertisers. They wouldn't be dependent on a few big advertisers. If an advertiser came to wikipedia and asked them to change a page, wikipedia would just say "no", publish the details to make the advertiser look like a douche (cue internet witch hunt, boycot naming shaming etc), and not care about the 0.000% temporary drop in revenue.