It's not even EVERY. SINGLE. DETAIL. A medium-two sized summary of who they are and why is enough. What we DON'T want is 2 sentences. At this point, the only thing that captures a character is old lore.
Look at "A New Dawn", their newest cinematic.
Characters included had mass amounts of lore that had at least explained something about who they are and why. It might seem irrelevant because that was nearly a year ago, but really, they obviously had no trouble capturing characters in the ways of their old lore, and to be honest, the more information you have on someone, the more you can expose and express. Not only to bookworms, but to every form of fan work, it helps capture certain personalities and characteristics.
I doubt that many people know about Lux's real story. But some fine details like that are what animates these characters. It might paint a small tinge of sadness that you cannot unsee. It's subtle, I know. But it will always express and expose more personalities and characteristics, new or old, than a cinematic. Both are good for their own purpose, but none is a replacement for the other.
And finally, the answer to your question:
"Yes, you love having strong, rich lore in a game, I do too. But is it REALLY required? If what we're getting is GOOD, just not ENOUGH for you, is more REALLY needed?"
Needed?
Nothing here is needed. Most of what Riot does is not needed. However, Riot is pretty darn aware they harbor the most played online game in the world. And you're going for needed? For every other department, their stuff is mad insane. Artwork, models, cinematic trailers, animations, voice acting, music, you name it, they got it. And they work dead hard on it too. Watch Frequencies. Do you see the dedication they have, the time they spend, the intensity of the effort they exert onto something that isn't even needed? Go watch it, and you'll be amazed about how much the characters matter to them. Now imagine them making a soundtrack based off of someone newer like Rek'Sai. It will, I assure you, not be as good as any other champion with the more compact, rich, old lore.
Now imagine, if the lore team put that much effort into things. You see the big flashy things like the Music and the Cinematic Trailers all with their own behind the scenes and how hard they worked on it, how long it took for them to pull out such an extravagant piece of art that millions of people will marvel at, but lore doesn't need those. But Lore expresses its own behind the scenes. Within every word, there is a thread that will slowly unwind a curtain and entice you behind those very scenes. Or at least it's supposed to. That's what lore is, a more subtle, yet brilliant form to express the development of a character.
Now, I ask you:
Why does it matter that any beyond the extent of gameplay is reinforced into a richer form of itself? Well, I'll take that up myself.
It's because nothing right now is as good as it was before. It's not the need, the yearn for books and books, but what we've changed to, what we've devolved to is a much lesser version of itself.
I'll just leave this here, and tell me which one you'd prefer.
Skarner's New Lore:
Skarner, the crystalline guardian, defends the entrance to a realm deep beneath the Shuriman wastes. The few who survive trespassing his domain describe a creature of terrifying intelligence, anger, and precision. What this merciless creature protects, no one knows.
Skarner's Old Lore:
Centuries ago, there lived a race of creatures as ferocious as they were wise. The Brackern were unusual beings blessed with the earth's primal magic, embodied in the form of crystals. Their kind knew of an arcane ritual to bind their life's essence to a crystal, communing with the magic embedded in it. Wit h this power, the Brackern thrived in the Odyn Valley, protecting both creatures and crystals alike. Despite attacks from those looking to claim the primal magic, it seemed that nothing could get past the Brackern's defenses. Nothing, that is, until the devastation of a Rune War.
A vicious battle was fought near the Odyn Valley and unleashed the chaotic magic poisoned the crystals. The Brackern began to grow sick and die, and no amount of defensive magic could reverse the effects. To avoid extinction, their only course was to hibernate underground until the wars stopped. The strongest, most intelligent Brackern hibernated closer to the surface and awaken first and assess the world's condition for the return of their race. The recent mining operations and violent escalation in Kalamanda were enough to waken the first of these vanguards. When Skarner burst to the surface, he lashed out in anger and confusion. Reason eventually won out over his anger when he realized that the agents of the League who found him were simply functionaries. Skarner was invited to the Institute of War to share the Brackern's story. In return, the summoners told him about the history that his kind had slept through. Much had been done to curb the unchecked use of magic since the Rune Wars, but it was obvious that the world was not yet safe for his kind to reawaken. For now, however, there is a place where Skarner can use his power to change the world into one to which his kind could eventually return: the League of Legends.
"The humans have not yet learned to control their magic -- what once was Kalamanda is now a crystal scar on the history of this world."
--Skarner
Are all very much lore-less champions. (With the exceptions of some working theories that may apply)
They have "theories" and snippets of character, but lack any real reason as to have that character as they have no story to have shifted them that way.