Not that that's inherently the end of the world or something, but it's a concept Riot clearly doesn't understand.
A few champions are straightforward like Yasuo. His kit gives him all the tools he needs to function inherently, meaning his best runes and items is just raw combat stats. IE + another crit item is core, into lifesteal and/or Sterak's followed by Last Whisper. His best keystone 90% of the time is the most popular one for other champions (currently Conqueror), not to mention Yasuo is incredibly popular himself meaning that everyone and their mother has seen what build Yasuo most favors first-hand at any given time. Couldn't be a more simple champion to build for right now.
Others are like Renekton. Let's see...he has damage abilities with good AD scaling and a chunk of potential healing attached to it, an ability that procs multiple on-hit effects at once and an ult that increases his max HP sizeably. He likes CDR for his abilities as well as sustain and tankiness to make use of their survivability, plus armor pen to continue to deal relevant damage and mobility to function past laning phase.
Renekton loves - nay, needs - armor pen and mobility, and then afterwards he suddenly wants sustain and tankiness so he can put his health pool to use in teamfights. Once he has BC and/or other arpen items under his belt, suddenly lifesteal is much more potent and therefore by extension AS. Sterak's is extremely good on him as well thanks to its synergy with his ult but is incredibly durdly until he gets some form of large bonus MS.
What form of mobility one wants most on Renekton is up in the air. Youmuu's + BC is a wild, loony core that forces him to ult on cooldown and blow his kit on opponents only to b, rush back out and do it all over again. DMP is an alternative for mobility but offers no AD which he needs in order to function early on. Relentless Hunter is a solid way to make his roaming more powerful but involves a commitment to Domination. TP helps a ton but reduces his kill potential in lane which he otherwise excels in.
Besides BC, DD is another very good core item on Renekton and pairs well with Titanic but also demands some form of armor pen, leaving LW to be an optimal in-between piece for the two items as Titanic gets significantly better the longer the game goes on as well. Sunfire is also deceptively good as he can pretty much act like an annoying fed tank in laning phase with it. Even Wit's End is good on the croc at all stages, so long as the MR can be put to good use.
Besides DD, BORK is another very good sustain item on him as well, but has the cost of being tough to build in lane coupled with being uncomfortable to build later on over defense. Sanguine Blade is also fantastic but simply can't be built first remotely comfortably - it does well as a 2nd or 3rd item. Wit's End also provides notable sustain while under half health and a bit of mobility to boot.
Renekton is known for wielding Conqueror and PTA but can also use Grasp, Electrocute and even Phase Rush, Arcane Comet and at one point Dark Harvest and Unsealed Spellbook very effectively. Do you see where I'm going with this? **This is only one champion's potential buildpaths in a game that will soon have a grand total of 150 champions.**
This is why players deserve to be informed what's worth getting on what champion and when in-game and with pinpoint accuracy. Their respective winningest items and the order they are built in - based on keystone and summoners, mind you - should be on display in the shop at all times. There's really no excuse after making such drastic changes to the game over the past 2 seasons to leave newer players or players that don't either pick FOTM or literally stare at OP.GG remotely in the dark as to how to win with their picks. It's frankly ridiculous that smart item builds haven't been implemented already.