There are many places on Wikipedia that explain exactly why.
The short answer is, because it costs time and effort, and a little bit of money, to maintain all those articles, and they cause more problems than they are worth.
Amongst other things, looking out for vandalism and misinformation. Also managing categories, infoboxes etc as they get updated. There's quite a lot. Just look at the history of any article.
The short answer is, because it costs time and effort, and a little bit of money, to maintain all those articles, and they cause more problems than they are worth.