because they don't have the man power to address issues on the older os versions.  it is also not in the interest of share holders to fix them. they would rather people get the new version.  the main issue though is their interactions with new architecture, especially thru the internet.  the new os systems are so bunk that they create new security risk when operating in the windows 7 and 8 environment.  such as outlook, teams etc.  that is why it is so easy to attack certain companies that are operating the corporate versions of those oss.  especially when you have widely available tools such as kali linux that makes attacking these systems easy enough for a child to do.
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