I can’t speak for everyone, but my guess is why people don’t like them is because it feels the value of the content and reward that is being boosted is being lowered to the point of it being useless.
To point it better. Let’s say that there is an achievement that requires you to kill the final boss of a raid on Mythic and you really want to get this achievement. You know may already know, this task might not be easy for that person as they need to be part of a guild that is able to clear up to that boss which generally involves understanding mechanics of the fights, understanding their class, and having the right gear. So you get yourself into a nice guild, you bond with them and get gear, group up with them and raid with them every week and finally after some time and most likely bashing your head against the wall to get all the other bosses, you finally make it to the final boss to work on them and after a while of working on them, you finally beat it and you get the reward of the achievement to show off to others your hard work.
Now, let’s bring in the person who is buying the boost.
Person buying the boost comes in with a large stack of gold. This person worked hard for that gold ((Or just bought it AH for $20 bucks)). They don’t really have time to spend playing the game as much as you do and so they aren’t part of a guild, nor spends the time to learn all the mechanics and and ins and outs of the fights, or doesn’t fully play their class to level of those who shoot for Mythic content nor do they do Mythic content with anyone. So this person requesting the boost dumps this gold down and asks this guild who has been clearing all Mythic bosses every week to bring them along for this large gold they have just so they can obtain this same achievement you just obtained and they agree and the person gives the gold to them and they bring him and in most cases, they just have the person die in the fight and they sit there watching them clear the boss and at the end of day, they defeat it and they got the achievement.
Now you have yourself sitting there with this achievement that took you a long time to achievement based on your ability to join a guild, understand all the fights in that Mythic raid and etc. You worked hard to get that achievement that not many may have and then you have Mr. Iboughtaboost who now has the same achievement as you showing it off to others they have it.
By doing this, it kind of makes some people feel like the this reward ((the achievement)) wasn’t very important or valuable because someone could have just drop tons of gold to obtain it, without going through the same kind of work you did to obtain it.
Kind of like you and someone being hired for the same job and you put in extra work and overtime to complete your work and you did excellent for it and got promoted, then the other person may have put in maybe 2hrs of work ((and this 2hrs of work was most likely copying some work off of someone else)) and got promoted.
It’s just the feeling of the reward you put in is just no longer there when you find someone who bought a boost to get it. Feels like “why should I put in that work now when someone else standing next to me is going to get it by just dropping 500k gold?”