Because Relics basically have a random chance to drop from Loot Chests, Bosses, Events, etc...
They also have random improvements, so there is no "Go and get this Relic" because the very best ones will either come for difficult content, or be randomly generated so there is no real means to farm them.
To add to the above, relics are pretty much what is replacing the weapons in the loot table, so there is nothing special about them. Every spec has 2 slots (3 with completed OH campaign) of different types and you just mix and match the relics to the correct slot type.
Meaning that in the loot table they take on the role of weapons, while in function they are weapon-only gems.
To add add to that, you'll probably wind up looking to specific spec guides to find the relics you're shooting for from say, raids and etc. not an overall guide.
i dont think that theres too much need for a guide relics come from pretty much everything you do in legion , and ofcourse the best ones comes from dungeons and raids so if you want to know who drop em check the adventure guide ingame and you are pretty much set . now there are some boe green relics than come from normal mobs in the world youll find em on the AH pretty much as soon as legion is out . other that that is just WQ ( world quest ) and dungeons / raids .
There are 10 different types of relics, each spec uses a total of 3 relics. A Holy priest will use Holy, Life, Holy relics while a Shadow priest will use Shadow, Blood, Shadow. Instead of weapon types like Two-hand, maces, staves and so on bosses will drop a specific relic type that only a few specs can use.
While the least used relics are Fel/Holy with only 5 uses.
Every relic also give a specific bonus artifact trait because every spec has a different artifact tree the artifact trait is spec specific so you need to look up what a relic will do for your own spec. Relic boosts your artifact weapons ilvl. How big a boost depends on the ilvl of the relic so raid relics gives a bigger ilvl boost than dungeon relics. Some specs may have a very powerful trait so they may pick a lower item level relic if it gives the right bonus trait.
When you first get your artifact while leveling you can only use 2 relics but once you finish your class order hall campaign which takes around a week you can use a extra relic. Unlocking 3rd relic gives a massive boost to your artifact's power so you want to finish your campaign as fast as possible.