Because directly citing Wikipedia isn’t how you’re supposed to use it in that sort of setting. What Wikipedia is best-suited for in actual research is being a first stop:

• Get some basic understanding of the subject via the lead section.
• Potentially get some idea on if this subject has plenty of information about it before choosing to write about it (e.g. if you’re writing a high school or undergraduate essay, you probably want a topic that has a wealth of information about it, not to spend weeks trawling through archives trying to find whatever scraps you can).
• Edit: Because we try to make technical articles highly accessible, the way an article is written can also give you some idea if a subject is way out of your depth by seeing if you can even follow along with what it’s saying. This is important because a) it makes writing easier and b) there’s less risk of close paraphrasing when you understand a subject enough to put things into your own words.
• Find what’s hopefully a wealth of decent sources that will introduce you properly to the subject.
• Give you leads for what to search for elsewhere by giving you some key pieces of information about the subject.

I would even say that professionally written encyclopedias like Britannica aren’t really a good “end source” that you should be citing. You absolutely can use Wikipedia for homework, but you still need to have some basic understanding of how to evaluate the sources that Wikipedia uses. Essentially, we ideally give you a head-start, not a finish line.