Sometimes people would rather avoid talking about suicide, because if the person they plan to talk to doesn't understand what they are going through in any way, then that could just make things worse. The reality is that as humans we are alone and nobody else can experience life in the same way. Often talking can lead to invalidation which is why it's better to stay quiet.
In this world a large amount of people are against suicide and are ignorant of the cruel reality of this existence, they would rather deny the suffering in this world and avoid the truth. It's particularly not recommended talking about suicide plans as it can lead to others potentially interfering. Trying to prevent suicide will only ever prolong suffering. I do wish that we lived in a world where people could be open about their plans without all of the stigma surrounding suicide. If someone decides to leave this world then there is nothing wrong with that and it's a personal choice which should be respected.