I'm not sure what an MHTML file is (I found the definition to be MIME HTML..), but it looks like it's just a regular HTML file with MIME standards.
The reason there's no tool available is because not all MHTML files are alike. It just so happens that YOUR mht files probably all look alike because they're generated by the same program. Yours is a special case, and unless someone else has run into the same issue with the same program, it probably isn't documented how to convert your mht files to something Excel can read.
If we could see the underlying source code (possibly you could attach the mht file) so we can see how it's formatted, then someone might be able to help you put together a script to convert an mht file to a csv (comma separated) file, with which you can import that data in Excel. The idea is that you want to strip the code, and be left with the data.
Hope that helps,