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.

The intended audience of a research paper is not 'everyone'; it is other researchers in the same field.

As a comparison, consider things like car manuals, or legal documents. It would be possible to write these in more accessible language, but that would detract from the primary purpose of the document.

In short, because undergraduate students are not the target audience of research papers' authors.

Research papers are often written in a formal and technical language, which can be challenging for undergraduate students to understand. There are several reasons for this: First, research papers are intended for an academic audience, primarily composed of experts and scholars in the same field. The language used is often specialized and assumes a certain level of prior knowledge and understanding of the subject matter. This allows researchers to convey complex ideas and nuances more efficiently, without having to explain fundamental concepts. Second, the primary purpose of a research paper is not to educate or inform a broad audience but to present original research findings and contribute to the advancement of knowledge in the field. Researchers are often more concerned with conveying the significance and implications of their research to their peers than with making it accessible to a wider audience. Third, using technical language helps to establish the credibility and authority of the researcher. Academic writing is often evaluated on its rigor, precision, and clarity, which are all qualities that are closely tied to the use of specialized vocabulary and technical terminology. Lastly, research papers are often written with the intention of being published in academic journals, which have their own set of formatting and submission guidelines. While some journals may cater to a broader audience, many are geared towards a specialized audience, and the language used is often reflective of this. However, researchers are increasingly recognizing the importance of accessible language and are taking steps to make their research more readable for a broader audience.