It has a lot to do with hunter having had, what many consider to be, the most broken deck hearthstone has seen, in deathrattle undertaker hunter. Other reasons include the perception that it takes no skill to play. While midrange hunter isn't hard to play, it's on about the same level as midrange paladin and midrange druid in terms of skill. They have the same style of play, stay on curve, make favorable trades, push for lethal. If anything, from my expierences on high ladder, I would much rather play a hunter than queue into a freeze Mage and waste 20 minutes having them draw, freeze, clear, draw, freeze, clear, alex, burn. Face hunter is one of the easiest decks to learn, but isn't so straightforward to master. Having watched others play the deck, there are times when they will pass up on a trade they should have made, something which is usually not obvious. The deck has decisions they just revolve around how to play the early turns and calculating damage and mana, sometimes several turns in advance. Again, allow me to stress that I'm not saying it requires a high skill level to play at a rank 5 level, I'm saying it requires some skill to play it at a legend level.

I've played hunter on ladder too and decks like face hunter are not nearly as strong as many people on this subreddit believe they are. I played it during my climb to legend and some while in legend: 100 games 55% winrate. Control warrior 50 games 60% winrate. Handlock 75 games slightly above 60%. I would not consider myself a good control warrior player, and I feel I'm about average at handlock compared to other players in the rank 3-legend range.

I personally don't understand why people get angry at the class for, "just trying to kill the opponent as fast as possible." The goal of every deck on ladder (unless you're trolling) is to win the game. Some decks do it quickly and others take their time. I would argue that ladder would be worse if aggro was unplayable. Imagine having to grind ~200 games a season while playing a deck that takes 20 minutes to finish a game.

The class also evens the playing field. Newer players can put together a deck that can be competative on ladder which is something other free to play games lack.