Why do people keep saying ‘I am going to make a xerox of this’ when they are just going to copy a document? ?

Xerox is the name of a copy machine manufacturer. Their name because synonymous with "copy" because the machines were so common. It's similar to calling facial tissue by the well known brand name Kleenex.

who hoovers a carpet...?

Because in the beginning that is the only thing out there that would make a copy, so it became the term of usage!

Xerox invented the photocopier.  

Do you Hoover the carpet, use a Kleenex or a Band-Aid, take an Aspirin or put the trash in a Dumpster? 

you know that xerox is a generic way to say make a copy

  it's the same as when you say you want a kleenex when all you mean is a tissue

Have some fun and ask one of your coworkers to mimeograph a document for you.

Xerox is a brand name.  They were once the primary manufacturer of office copy equipment.  Kind of like Kleenex, the brand name has become shorthand for the thing itself.