Why do Muslims say "As-salamu 'alaykum" (plural -kum), even when only one person is addressed?

Question

Shouldn't it vary depending on the number and the gender of the addressee, e.g. 'alayka, 'alayki, 'alaykuma ? But I almost never heard the other variation. Is there a religious or grammatical reason for this, or is it just non-Arab Muslims unfamiliar with Arabic grammar?

Answer 1

The idea is that peace is upon you and the angels around. The belief is that we are never alone, we are part of a universe where humans and non-humans co-exist.