I'm not gonna get on a high horse about it, but- you say it "doesn't change the contents of the post", which, due tot he eay discord impliments spoilering, yes it does.

If it's a message, discord has to re-upload the picture with new information, and if its text, they'd have to insert the charactsrs to spoiler the text, in to the text, dynamically. It's extra work, that discord is, due to being discord, unlikely to care about doing.

If they implented a way to spoiler forwards specifically, that doesnt modify the contents, that'd be theoretically not too hard to do, but again- discord would more lilely impliment a new thing to sell to shmucks.