Answer:I'm only a year or so in to learning but believe it's more or less a partial phrase.
こんにち is like "this day" and は is just the particle
So こんにちは is like "as for this day(it is 'insert unspoken words')"
Same with こんばんは "as for this evening"
If you were to say to someone on the street "Beautiful day" which is just an adjective and a noun but doesn't have a verb anywhere, you'd know they just meant "It's a beautiful day out, don't you agree?" and that it was a greeting.
The whole partial phrase thing happens a lot in casual speech. When someone asks あなたは "as for you?", it's typically asked as a question but doesn't have a か or anything about what is being asked. Context.
Explanation:
