it is both luck and skill but if you play enough skill will override luck.
But since you are in bronze you likely don't have a a consistently strong showing compared to other players in your games so you are there by skill.
Accept this and you might be able to improve and escape.
Signed a player who found their way out of bronze.
Andi f ou want an example of how it is not pure luck but loaded luck think of every play as dice.
An average player has 1 through 6
A bad player has 1, 1, 1, 2, 3, and 4
A good player has 2, 2, 4, 4, 5, 6
A challenger might have 3, 5, 6, 6, 6, 6.
So while anyone can beat anyone the good player will tend to beat the avrage and bad player most of the time.
Each team has 5 dice meanign that while the good player providing a 5 will help it will not make up for everyone else rolling a 1 on his team while the other team has 4, 3, 3, 1 ,1
Aka 12>9
But overall the good player will consistently make for a much better chance of winning if he plays enough games. So if his team were all rolling 2 that would mean is team would beat that described team above
13>12
So stop focusing on your team mates building warmogs on Riven first thing and instead focus on making your role better.
work on getting one of the 1's on your die to be a 3 and things should start improving for you.