Grammar - past tenses

Hello,
could anyone please explain to me which grammatical rule (or rules) warrants the use of the past simple tense after 'since' in the sentences bellow:

* We hardly recognized each other, because we hadn't met since we were young.(B.D. Graver. "Advanced English Practice. Oxford U. Press. 1990. page 85")
* We all knew he had been drinking heavily since his wife died. (op. cit.)
* He had been a soldier, since he was seventeen, and planned to stay in the army. (A.J. Thomson, A.V. Martinet. A Practical English Grammar, Fourth Edition. Oxford University Press, 2001. page 175)
I also found a sentence which, I would say, is more along the lines of what I believe or imagine I know about the use of the Past Perfect:
* His father had died four years before and since then Tom had lived alone. (Thomson, Martinet. p.176) By the way, can this sentence be transformed into the following one: "Tom had lived alone since his father died."

I would like to know why the Past Simple is used in the above clauses after "since". The coursebooks I cited only mention that such a tenses sequence is possible with "for" and "since" and they don't delve deeper into the matter. Is it some kind of a language praxis? Why is the Past Simple use after "since" not in violation of the rule that the earlier actions are expressed in the Past Perfect, as this what I imagine is the case with the word "since", which underscores an action earlier in time?
Sorry for all the mistakes I probably made. Thank you in advance for all the responses and explanations. Have a nice day.