Riot's item design is what has led to the current explosive damage meta.

Back in season 5, Riot found that they had a huge issue when it came to balancing champions: they could not determine whether champion imbalances were the result of the champion being too strong or simply being dependent on items which were extremely gold efficient. They way they chose to resolve this issue was to normalize the gold-stat efficiency across all or most of the items in the game, and make individual item choices much more about passives and inter-item synergies than raw stats. [5.22]

Because gold-stat efficiency differences across the items have largely been removed, Riot hoped that items would be selected on a case-by-case basis by their passives and the situation... but what really happened is that any actual or perceived difference between similar items led to people choosing the "better" one.

In order to force players to make more choices, Riot has incrementally taken steps to lower general utility across items and specialize them in an attempt to diversify item selection: we can clearly see this with mage items [6.9], support items [6.22], and marksman and tank items [7.9].

So what does this item philosophy mean for the overall game state?

Champions can generally be grouped in one of 3 categories in regards to how they interact with items: a) Generalist champions (fighters, bruisers, hybrids), who are flexible champions who use many different kinds of items to adapt to individual situations and matchups. b) Specialist champions (marksmen, burst mages, assassins, tanks), who typically stick to one build-path for items, as their focus is more about fulfilling specific roles than adaptability. c) Support champions (supports, some mages, some tanks), who have large intrinsic utility and are usually not directly affected by changes to the item pool.

Riot's hyper-specialization of items has led to a number of things:

1. Individual item purchases are not very good.
2. Because (1), to maximize gold-value items must be purchased along specific paths to take advantage of multiplicative synergies across items.
3. Because (1)(a), generalists cannot be flexible; because (2), they must buy items as specialists do.
4. Because (3)(a), generalists function as worse specialists, and are largely non-viable.
5. Because (2)(b), damage focused specialists are greatly favored and multiplicative scaling only furthers their gameplan: dps champs get more dakka, burst champs are burstier.
6. Because (5), specialist tanks are made tankier.
7. Because (5), supports can get killed much more easily; because (1) it is difficult for supports to splash defense.
8. Because (4)(7), specialists become the most important heroes in the game.
9. Because (2)(8), items are even further specialized making specialist classes heavily dependent on the power of the item chains made for their class.
10. Because (9), entire categories of specialist champions are rendered overpowered or useless based on item balance, and the game becomes an arms-race between between dps vs tank vs burst.

It's actually quite interesting just how far they've jumped down this rabbit hole as all of their balance changes seem to only have exacerbated this problem: Individual champions get nerfed due to item synergies, but then are left weak when those items themselves are nerfed. Older, less focused (read: generalist) champions find themselves non-viable in a field that increasingly favors specialist roles, and are reworked into champions with much more specific roles assigned. The old mastery and runes system which provided a decent chunk of raw stat value to champions to smooth rough edges was replaced with a combined system that necessitates even more specialization on the part of champions, with its mandatory specific tree and keystone selections.

Based on the above I think that Riot's attempts to diversify item and champion selection are largely a bust, and that the current state of the game was inevitable. I do think this problem is fixable, but Riot is going to have to start making room in their game for more generally powerful items and options if they want to resolve this, otherwise they will be perpetually locked in a cycle of playing balance whack-a-mole with entire classes left by the wayside.

tl;dr; Riot's item design philosophy has created a situation where only specialized characters are truely viable, and entire classes of champions are either too strong or useless depending on the power of the synergistic item paths that they are _forced _ to build.