Because:
1) That's also often not the case
2) Even with sources, it's subject to however particular people (goodness knows who) decided to phrase it
3) There can be issues with the sources that were chosen versus those that were omitted
4) It's subject to someone misinterpreting something, vandalizing something, or other such factors
5) "Encyclopedias" don't make good "sources," regardless of which "encyclopedia" it is
6) It's just not a good habit to be in
Among other reasons.
1. Do you have any evidence to support this? Anecdotally, it is often the case for me, but I can't prove it one way or the other.
2. If you're concerned about this, then click-through to see the original context. That's why it's there.
3. What does this even mean? This could apply to anything anyone says or references. There are always other sources that could be quoted out there. Or are you saying there's some finite, specific list of resources that must be used because anything outside of that is not trustworthy and shouldn't be used as a citation?
4. See #2
5. For what? A dissertation or a master's thesis, of course you wouldn't use wikipedia for a source. Answering a question on a forum? It's often perfect.
I understand the angle you're coming from since you seem to be an academic, but not everything requires that strict of a standard.
edit: basically, it's always up to the reader to do his/her due diligence on a source posted online and decide if it's trustworthy/valid, whether that's wikipedia or somewhere else on the web