Because automating homes is not their goal; profiting when others automate their homes is their goal. They appear t believe that they will make significantly more profit by selling home automation tools and the things that attach to them than by carrying some other company's home automation tools in their catalog in addition to their own.

Fraud concerns most likely. They have an api they support for hooking up to Alexa.

Why do you assume they aren't working on it?

Big companies move slowly. I doubt Amazon could roll something like this out without 5+ years of development - possibly longer depending on how many times the department gets reorg'd.

Because they want people to buy Alexa?