I think its a convenience issue as well. Most of the time these things happen after a ready up.
And with the delays this game has, the actual timer might just be started too late, so the pull was already committed.
The best solution here would be if the ready check could automaticly also trigger the timer (so after everyone is ready, you have a 10s countdown). If people know this timer gets automaticly started, they are more likely to wait. To have it always would be meaningless since sometimes its just used to ensure everyone can face the start of the battle (as sometimes the start is significantly better for dealing damage because boss mechanics start later on), so in these cases people just can pull whenever they like.
But in the end, we are talking about people... it never will be perfect.