Why does amazon spam me with something I bought already?

[DunbarPappy®ϟϟ]

When using almost any service (Amazon for instance), you 'agreed' to their Terms of Service, and almost certainly they mention the right to share your activity with "3rd parties" (or 'affiliates', 'partners', and likewise) in order to generate revenue for Amazon (or whoever).
They accomplish some of this with 'tracking cookies', or perhaps Amazons own databases.
This is the current revenue model of the Internet.
You may have to re-examine your Amazon account settings and see if you can "opt out" of that kind of arrangement, and it helps if your browser is set to decline "3rd party cookies".
Exact steps depend on what browser is used.