Because the whole problem of the game is feeding.
If you compare a garen with a kata, a jhin, a sylas, out of the blue, and you adjust their data based on this, it's cute but... It never happens that way.
In reality (in my experience) you have feeders 9/10 games (sometimes multiple ones in the same game), so you will never face an equal opponent.
Let's take an example : I was playing poppy and having a great game. By minute 7, I had emptied my opponent's north camp twice, had 0 deaths and 2 assists. Except my gp top was feeding a pantheon and zed mid was losing to a veigar. By min 15, GP had died 3 times, zed and soraka 4. I was 2/1/3. At this point, I could somehow outplay pantheon (it was tricky though), but veigar? he just had to press R on me and I was dead.
On one side, you have a poppy who needs to land an E in the right angle, several Qs and auto attacks, to kill the veigar, on the other side, a guy who kills with one point and click attack. Now that's a TOTALLY DIFFERENT game than the perfect world where all champions are equal. And there wasn't much I could do : I kept pinging GP to play safer, I kept trying to gank him (the first pantheon's death was from my gank), but GP always died before I could even reach his lane. Then pantheon could freely roam mid while GP was content with raising his cs solo on lane. Except now veigar was fed too. And obviously my 0/0/2 poppy wasn't going to beat a fed panth and veigar hard pushed under tower by our mid. So the game was only balanced at champ select. As soon as GP and zed started playing, they made it impossible to win. Not even mentioning the 1/12 soraka.
That is exactly what game designers need to fix. They need to make sure every player has a chance to win even if match making didn't balance teams correctly