Some have. They're also usually the ones willing to have constructive discussions and have some respect for opposing opinions even if they don't agree with them.
But then there are the ones that are a flood of negativity, with the frequent "this game is terrible, all the devs deserve to be fired". It's very hard to see those individuals as being passionate about playing the game.
The problem with confining the positive remarks to Discord or Twitter is those remarks only reach those that share the specific social circles where you post. Anything said in Discord is unlikely to reach the developers. Anything said in Twitter is likewise unlikely to reach them unless one of the common FF specific hashtags that Community Managers look for is used.
Things posted here in the official forums can reach anyone who pulls up the forum. No account or active subscription is required to read, only to post.
If someone is only a hater, yes they should go away. If someone enjoys the game but has concerns, then let's have a constructive discussion here instead of one that amount to "the devs are all incompetent and have no idea how to make a good game, fire everyone".
This is a good explanation of why most posts are going to be negative. Humans don't have a tendency to talk about the things they like or that meet expectations. They do want to talk about what doesn't.
But just because we have this tendency doesn't mean that it's the way it should be. The problem is that it sinks us into a well of negativity that can start to overshadow the positives.
The game developers do need to know what players are enjoying as much as what players feel need to be improved. If all they're getting is the negative feedback, that's got a high potential to impact their work performance. Why bother putting effort into anything if no one appreciates what they're doing?