Aside from what others have mentioned, the wonder is actually that there's something like Let's Encrypt itself. It was a huge lift by the LE founders/engineers

Aside from what others have mentioned, the wonder is actually that there's something like Let's Encrypt itself. It was a huge lift by the LE founders/engineers/benefactors to pull it off, and they had a larger mission to make the web more secure for end-users. Document signing has no such larger mission, so any players have to do it either for money or for the lulz, which won't get to LE benefit and scale.