Because of a simple yet very effective technique I call: "Asking them". I suggest safety tools for each new group I DM and to this date, all but one group have been open to the idea but after a quick discussion every single player told me that they see absolutely no use in having them and that they will let me know if they ever feel like the topics are getting too rough for them.

BTW, the one group that still has them active pretty much forgot about their existence. I'm a player there and as far as I can tell, the GM is the only one who really wants them.

It all comes down to group composition. If you're comfortable around the other players and the GM and know that you can just say "stop" whenever, then safety tools add nothing to your experience.