Alby does use something called a Master which is a 12 mnemonic words which then lets you create multiple keys for different purposes like Nostr, BTC, LN etc but they need to extend it to create multiple pairs for same purposes as well which will come. Key rotation is complex for users so signing a message offline is better with one pair might be easier. Also you only need ti remember 12 or 24 master words which can restore all your accounts.