I am using default installation and dracut includes resume module without any manual intervention and system performs resume (without LUKS).
You understand the difference between 'UUID of LUKS container" and “UUID of decrypted LUKS volume”, do not you?
It sounds like dependency problem between systemd services. Normally resume should be performed as the very first action, but to resume from encrypted partition it has to be decrypted first. Which is why I asked you from the very beginning - did it work without encryption?
First, swap partition is not “mounted”. As for your question - because decryoting of LUKS partition needs human intervention and it can take arbitrary amount of time.