I understand what this error message means and I know how to fix it. I just want to know why LibreOffice doesn’t include the required .dll in its setup file. Can someone explain why this is hard to do?
It includes the DLL in the installer. But there could be a bug in a specific version, where the installer was built incorrectly. So - the question is not “why this is hard to do” (it isn’t), but “why don’t you tell which version gave that problem to you?” 
Its version 25.2. I realize now that its a pre-release version. i downloaded it from softexia.com, which usually is pretty good at posting only the latest release version. The .dll error will probably be fixed in the final release version.
Why would you do that? It is not the official site, who posts it? Have you reported to them? Have you run an anti-virus check over it? Checked the hash against that posted on the LibreOffice site?
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The pre-release has been available on the LibreOffice download site for a while (now on version 25.2.0.3) and I haven’t seen those sort of problems in it.
You are normally best advised to download the stable version. I believe that 25.2 will be a main download very shortly, see Release Plan 25.2 - The Document Foundation Wiki