It’s because they use FCM for sending push notifications and you need Google Play Services to use FCM. And Signal uses push notifications to know when to wake u

It’s because they use FCM for sending push notifications and you need Google Play Services to use FCM. And Signal uses push notifications to know when to wake up and fetch data. Otherwise, it has to poll and do other ugly stuff which drains your battery quicker or makes the user experience much worse. Signal’s taking a pragmatic approach that’s the best for most people, but not everybody.