two questions regarding the spellbook of the wizard, the pact of the tombe warlock with the book of ancient secrets invocation and the ritual caster feat

1. since these three use very similar mechanics, all letting you cast rituals from within the book as rituals at-will without preparation, all three costing 50 gp and 1 hour per spell level to write something into it and all being specifically special books, so why do they not all refer to a single rule from which other rules can quickly refer to if they want it to apply to all of them?  seems like that would be very convenient from an design perspective
2. would there be mechanical benefits from physically scribing spells you gain from multiple of these sources into the same book? for instance if an divination wizard with ritual caster has all their cleric rituals within their normal spell book, would they get the discount on spell price when scribing augury or divination into the ritual book, as they technically are putting an divination spell in written format in the same book as their spell book, even if the spell itself has nothing to do with the spell book mechanic? If i wrote my wizard spells into a book of shadows, would my spells still be in there if my book got destroyed and i used an ritual to gain an replacement book of shadows from my patron (or hey does the replacement book of shadows include all the rituals you lost in the first place)? 

Quote from Lyxen>>

I'm a bit lost there, by what mechanic would a wizard have access to Cleric Ritual Spells ? As far as I know, he normally would not.

And again as far as I know, only the Warlock can write (and cast) rituals from all classes in his Book of Shadows.

 i am talking specifically about the ritual caster feat from the player's handbook, not to be confused with the class feature of the same name, that could allow you to learn ritual spells from the bard, cleric, druid, sorcerer, warlock, or wizard spell lists and store them in a so-called ritual book, and of those options a wizard who specializes in divination magic will find ritual caster (cleric) to be their best option as all the cool divination spells that clerics get and that wizards lack are all rituals. Basically what i am asking is "what if the special ritual book was the same book as your spellbook, would you gain the benefits of divination savant to copying spells into your ritual book since it is also your spellbook"

1) Because. That is just the way they decided to do it.

Quote from ArtificeMeal >>

2a) Would there be mechanical benefits from physically scribing spells you gain from multiple of these sources into the same book? for instance if an divination wizard with [feat]ritual caster[/feat] has all their cleric rituals within their normal spell book, would they get the discount on spell price when scribing augury or divination into the ritual book, as they technically are putting an divination spell in written format in the same book as their spell book, even if the spell itself has nothing to do with the spell book mechanic?

2b) If i wrote my wizard spells into a book of shadows, would my spells still be in there if my book got destroyed and i used an ritual to gain an replacement book of shadows from my patron (or hey does the replacement book of shadows include all the rituals you lost in the first place)? 

 2a) No. The rules for wizard spellbooks apply to wizard spells. The subclass feature that discounts spells is for the wizard spellbook, and by extension wizard spells.

There are no rules against using the book of shadows as a wizard spellbook however, so that is fine. Just remember that even though they are the same object, features that apply to wizard spellbook and features that apply to warlock book of shadows won't overlap.

2b) Huh, I never looked at this that closely, but: No, there doesn't seem to be any rules that say the replacement book of shadows copies any of the original's contents (warlock related or otherwise). I would definitely house rule that it does and would even allow the wizard spells to copy too.

Quote from Lyxen >>

2b) Huh, I never looked at this that closely, but: No, there doesn't seem to be any rules that say the replacement book of shadows copies any of the original's contents (warlock related or otherwise). I would definitely house rule that it does

You do not need to house rule it for the Warlock, it specifically says: "You can now inscribe magical rituals in your Book of Shadows." and "With your Book of Shadows in hand, you can cast the chosen spells as rituals." and "If you lose your Book of Shadows, you can perform a 1-hour ceremony to receive a replacement from your patron." 

All of this refers to the Book of Shadows and what you do with it magically with your warlock powers.

Where it gets more muddy is what happens if the Warlock writes things in there that are not rituals, this is where you get into house rule territory.

 yeah but the PHB never outright says that the replacements has all the rituals you wrote in your book of shadows tho (book of ancient secrets)