Why is elo boosting bad?

Because those players didn't earn their rank and having skilled players on those accounts screw everyone else of their elo since the system already judged where they belong currently?

The biggest reason for me personally is that people who didn't earn their rank are getting to higher ranks - this is cheating others of the feeling of accomplishment when they get to a certain rank since they could have just as easily paid for it.

However, the biggest real reason is that boosted players ruin games. If you have a Bronze/Silver player who doesn't know the basics of the game playing in a Plat/Diamond match, you end up having a really shitty and boring match - the Bronze/Silver player goes 0/15, the enemy team gets super fed, the game ends quickly without any real strategy.

It doesn't matter if the Bronze/Silver player is on my team or the enemy team, easy win or easy loss are both boring. I care less about winning and more about having an interesting and close match.

Edit: Also, please don't downvote the OP. He's asking the question out of curiosity in an objective manner.

Because it's cheating?

Same reason why smurfs technically aren't allowed. You have high elo players tuning other players fun.

Whats elo boosting?

If a bronze player gets put in diamond he will feed every game and ruin the game for everyone else.