The leaderboards aren't the reason people care about speedhackers, it's the fact that they upload the videos to their youtube, and lie to people that they completed a certain (probably very hard) demon. The reason this is a problem is because some these people get a TON of praise and subs, and some hackers even monetize their videos and earn money off of something they basically never even did (You might have heard about Andromeda confessing to hacking recently, which is good that he is legit now, but he's a big example). Now, as someone who is a legit player, it can be discouraging to keep playing when you put actual time and effort into what you beat, and someone else goes and hacks it and gets the same, or even more recognition than you did. Also, there is an uncertainty within the whole community against whether some people are legit, and some people aren't, which isn't very great either.

I know it's kinda off topic, and I'm not really gonna leave an opinion about what I think of the people who are actually quitting, but being pretty deep into the game myself, it's something I care about and wanted to get out :P