Why are research papers written in language that's difficult for undergraduate students? As an undergraduate student, I find it hard to understand research papers on any particular subject.

Why don't researchers use simple language for their reports, so that everyone would understand?

Update: I will leave this post as a general discussion on this topic. Feel free to edit question.
In short, because it is difficult to express something concisely, and precisely in language that any undergraduate can understand.

Conciseness is required not just because without it every report would be inconveniently long to write and to read, but because it would be harder to understand.
It would be harder to understand because the jargon neatly encapsulates a bundle of concepts (e.g. its definition and related properties) into a single concept.
And we can only be thinking about so many concepts at a time.

Consider the statement about the Stone–Weierstrass theorem:

A mathematician might say:


  Polynomial functions are dense in C[a,b] ⊂ (ℝ→ℝ)


To expand out the math, so that one does not have to know the notations on gets:


  Polynomial functions are dense in the space of continuous real-valued functions defined on a closed interval.


But still perhaps the word dense is beyond the understanding of an undergrad.

So let's expand it to not use that:


  For every continuous real-valued function defined an interval; then for any positive real constant one might care to define, a polynomial can be found such that for every point on that interval the absolute difference between the value of that polynomial and the value of the real function at the point is smaller than the constant.


So that is how much most space it took and how many more ideas one has to keep track of for that fairly simple use of jargon.
When thinking about such a problem rarely is the mathematician thinking about what is going on with the distance of points in a hypothetical polynomial.
They are just thinking "it is dense".

Now imagine expanding all the terms in the generalized version of the above:


  Stone–Weierstrass Theorem (real numbers). Suppose X is a compact Hausdorff space and A is a subalgebra of C(X, R) which contains a non-zero constant function. Then A is dense in C(X, R) if and only if it separates points.


(This last is a direct quote from the Wikipedia page on the Stone–Weierstrass theorem, the preceding quotes are not, though are to some extent paraphrases.)

Then to go the other point on preciseness,
There is a really high chance someone is going to comment on this answer saying that actually my statement is not quite correct, that I've not fully captured the definitions in my explanation 

While, yes, every paper could repeat some introductory information,
then that would inconvenience any reader who is looking to find the core idea,
since it would be drowned in a sea of background material.

And you might say that "this answer is hard to understand, in the expanded form, you did a poor job at making it understandable to an undergraduate.",
and I'ld say "Fair enough; I'm not great at making things easily understandable."

And that statement holds for most other researchers too.
Not what most are good at -- it is why there are specialists in scientific communication.