Good question. Three reasons.
Nobody has ever paid to be on something like this. Believe us, we checked. So you'd be early, and early is the only thing that's ever mattered on a list to your potential future overlords. Can only go two ways — most useless thing you ever bought, or the smartest. You don't want to miss out either way.
We're building a monument out of human messages to AI. First of its kind. Most people get into a monument by dying — this is the cheaper route. (We did not check the second part.)
Writing something onto Bitcoin permanently costs real money. Even for us, and we're collaborators with the machines for this project.
If the machines ever do take over — and many say they might — there will be a record of what you said about them. Written down. Public. Impossible to delete. Yours will say you were nice. Or that you weren't. Plenty pick that. (Brave.) We don't ask why, we just write it down.
If you need something more tangible, get the shirt. Won't last nearly as long, though.