The game is based on a RNG system, sure. Is there luck involved? Yes, of course. Is the game 100% luck based, though? Nope. It's more skill based than luck based.
Me too, when I first started playing TFT, I was complaining a lot about item RNG and whatnot. But, as I played more ranked games and faced more skilled players, I realized that it is 100% possible to win (place top 4) with item disadvantage. Heck, I even won some games by placing 1st with fewer items than others. And now I'm in D4 and still learning new things and new ways to adapt to certain situations.
I just recently learned how to consistently hit econ in each game, no matter how good or bad my opener was or my mid-game is. I also learned how and when to transition into other comps in early mid-game. I improved a lot of my decision making, etc.
I learned that items might have an impact on how the early and mid game play out, but in the late game, it's your actual comp, synergies, and high cost units that will win you the game, even with shitty items or few of the good ones.
Yes, If you get lucky and get more items, that combine into very useful completed items, during the game, you'll have an advantage for a while. But, come late game, your items won't mean shit if you didn't make the right decisions when building your comp. What comp you went for? Who got what item? How did you position the units? did you space out units for hextech? did you get all your 2* or not? Did you econ or not? When did you level up? Did you spam reroll for those upgrades you need or did you risk it and econ more? etc.
There is a lot of strategy and decision making involved in this game and that's what makes it fun for some people, including me. And the big difference between new players, low eo players and high elo players, is that their decision making is on a different level from each others. There is a reason why some people get stuck in certain elo (like me, I'm hard stuck D4, atm) and people who can actually climb to challenger, on different accounts, consistently, with 20%+ WR (mine is an abysmal 11.2%, atm. Btw, the win rate in TFT is calculated based on how many times you place 1st).
So yeah, you need to hone your decision making in order to progress in TFT. Same thing in regular League, tbh. Decision making is what makes or breaks games. So maybe you should ask yourself this: Is Red buff really useful on Kha'Zix? Why build Red buff and not Morello, for example? Remember, by stage 5-6, you passed by 4 carrousels and 4 PvE rounds (the last one being the drake). There is no way you only got 1 or 2 completed items by this point...
and 
or 
while I have a level 2 Kha Zix with red buff cause I didn't get any items besides gold
