If anyone can edit wikipedia, what's stopping someone from just defaming or erasing every page?

The site has people monitoring for stuff like that. if it happens too much, the page will be closed to editing

Backups of pages are created every time they are altered, so a simple erase or clear defamation can quickly be rectified. In addition, a clear defamer will often be banned from further edits. Of course, there are ways around such bans, such as IP maskers, but why devote so much energy to a process that as mentioned can easily be reversed?

lol ppl do it all the time but there are ppl who manage the site and they have backups to restore it, also some pages are closed to editing

Its pretty right winged site in general.
Heck FOX news went in and did alittle adjusting.
They fixed alot of stuff, but in reading some of the politics, you can see they lean alittle to the right.

Plus, its extremely easy to mask as another person.
And be careful of the spyware.