Because they have "voice assistants" that have to be always on, always transmitting, because the software that recognises your words on your mobile phone needs help.

Facebook has access to your mic if you ever use it for its voice com functions (do not do that) and do not explicitly remove the permissions to access teh mic (do do that).

They have been caught several times. Thing is people give them permission to record through the mic so it is legal.

Do not confuse legal with good, it is evil.