Then tell them that's not your problem, it's open world content available to everyone and they're welcome to take it up with the GMs if they don't like it.
As someone who conducts trains, I'm tired of the organized hunt community acting like it owns everything hunt related. We don't. Yes, we want people to come together to have a good time killing stuff but it's fine to limit it to just what's available at that time. If someone else gets some of the marks before the train is formed, good for them. I genuinely hope they had a good time.
The people I really hate are the ones who wait around for someone to organize a train then spend their time disrupting it. If you don't like how the train is run, then why did you come? If you wanted to hunt on your own terms, why weren't you out there looking for the marks as soon as their spawn windows opened? That's public information with the various trackers being used by the community. No need to come bully a train that's intended to be a pleasant experience for those attending.
The mount achievements are intended for those who make The Hunt their end game and spend the majority of their time hunting (or those who have nothing in their life to prevent them from playing 20 hours a day). I know a lot of dedicated hunters who have all 3 mounts because they love hunting. I know a lot of hunters (like me) that don't because we also like to spend a lot of our time doing other content even if we're still out there scouting, conducting and spawning a good part of the time. I don't even have my Centurio Tiger yet (though I'm slowly creeping up to the end of the grind).
It's fine for achievements like that to exist. The game has plenty of other mounts for players to earn through other types of content.
SE has stated they are not willing to do that. The amount of time and effort it would take them to convert The Hunt into a FATE style system would come at the cost of a significant amount of new content they could otherwise be making for the game.
People just need to learn they're not supposed to make it to every hunt on their home world let alone cross world. If you want those achievements and rewards, then become one of those who are doing the spawning and the conducting so you're spending more time actually doing The Hunt instead of trying to squeeze it in between the other things you're doing.