Good players are like athletes, bad players are steroids
At the time of writing this, working up to destroy the enemy nexus before the enemy destroys yours is like an athletic competition. The good players are trying their best to become stronger than their opponent, but their opponents get a chance for a heavy dose of steroids, the steroids being the bad players working alongside the good players, and the good players' opponents become significantly stronger with little to no effort. No matter how much energy or time the good players put in, they can't outmatch the overbearing strength handed to their opponents on a silver platter. After that, there's nothing left for the good players to do but admit defeat when they can't even fight back.
Now, what I'm about to say comes almost purely from what I've seen after binging this game on an addiction for the past 5 months, and most of you probably know what I'm about to say, but the snowballing in this game is just too heavy. "Steroids" propel a team to victory, often leaving the other team completely and utterly obliterated. This is because damage is too high, gold is too easy to get, and a strong lead is effortlessly obtained.
For a bit more detail, once your team gets ahead of the other team, beating them becomes easier and easier until you soon severely out-power them and your pockets are lined with gold. Now, because you are so strong and rich, they have a much harder time earning gold for themselves since your team's sheer power punishes them harshly whenever they reach for it. Meanwhile, you continue raking it in since they have no way of stopping you, continuing the unforgiving cycle of snowball. And it wasn't even that hard for you to get ahead since, as stated before, their one player that's doing poorly gives you more than enough to let you win, usually making the performance of their good players almost meaningless for anything other than stalling the inevitable defeat.
This alongside the paper thin objectives that you can shred while the other team can hardly approach them as well as the laughably low turret DPS plus their weak ability to tank anything, let alone defend a base or its team, makes turning a match into a nearly insurmountable feat. In some cases, turning is nothing more than a pipe dream, because unless your composition is made for turning or the other team throws hard, you can just forget about it.
Now I'm not trying to say that this game should be handing out participation awards so the kiddos don't feel bad and that a team should never snowball, but it should at least give the losing team a fighting chance or an opportunity to strike rather than just dubbing them "dead-men walking" and calling it a day. That does nothing but leave the players to wonder why they don't just go AFK. Back when I started playing, there were of course still snowballs, but they were usually caused by a wider failure among it's teammates, and recovering from them was somewhat manageable, because damage wasn't as high, objectives weren't as weak, good players could save the dead weight players, and gold wasn't constantly denied by the winning team.
TL;DR: Good players are not beaten by better players, but by worse teammates weighing them down and giving the enemy a strong, easy lead that grows into an unstoppable force, making the performance of the good players muted by the ear-piercing screams of their midlaner dying for the 10th time while they are given almost no opportunities to turn the match around.
I personally love this game and would even go as far as to say it's one of my all-time favorites, but I hate how it is in it's current state.