Why do papers not have a table of contents?
The purpose of an overview paragraph and a table of contents are different:


The table of contents serves to quickly find a specific section of a document so you can start reading there or extract a specific information. This is something that rarely makes sense with most papers, except if you start at the conclusion/discussion, which is however easy to find anyway. Even if you want to do this, finding the table of contents would cost you more time than it saves you, given the shortness of most papers. So my answer to your titular question would be: Because there is no need for it.

The main exception to this are long review papers, which however usually have a table of contents.
The overview paragraph serves to briefly summarise the paper and explain its semantic structure to the reader, usually somebody who is reading the paper from back to front. This serves to address certain expectations the reader may have had after the introduction (“don’t worry, we didn’t forget this important aspect”) and draw connections between sections (“our work in Section 3 provides us with a framework to evaluate our method in Section 4”). All of this is not possible in a table of contents.

Now, some papers just have a straightforward structure, and there is nothing about it that needs to be elaborated to the reader. In this case an overview paragraph would indeed be nothing but a written-out, tedious-to-read table of contents and should be skipped in my opinion.