Why does my Promise definition gets executed?

That is simply how promises are defined. They run their executor function immediately. It's in the spec: Promise(executor), step 9.

This is an instance of the revealing constructor pattern; reading that might help you understand.

That occurs because a promise will execute immediately and synchronously.

.then() add functions that will be executed when the promise is either fulfilled (resolve argument) or rejected (reject argument).

_{with info from comments by @Kirill Slatin}