Among the reasons is that it comes with great power and authority and therefore attracts the wrong people. It is really effective to slander people for example. Another reason is that an established source of truth is inflexible and hard to change from the outside even if it is wrong, because it is used as reference for the truth (circular logic).
Using Wikipedia as an example: Everyone can edit Wikipedia, however if your edit survives is determined by a hierarchical structure that involves people (with an opinion). It doesn't need a big conspiracy, but only a few people that select which people may act on their behalf. A concrete example would be the Wikipedian Feliks in German Wikipedia who is by now convicted of defamation (via Wikipedia) and still remains part of the "Politburo" of Wikipedia altho obviously violating the official ethos.
In regards to changing Wikipedia, try removing opinions (or just circular citation) from an entry of Feliks or Kopilot (or anyone of the Politburo) and look what happens. Most likely your account is gone, but if you're lucky your edit is just reverted.
Much of it is how Wikipedia is being treated. It is/was an excellent reference book for quickly looking up information about a topic - well, just like an encyclopedia, however it's bad as a source of truth.