Probably so people don’t have to scroll down further to have to read through it twice (once in the image and then again in the transcript), on the way to the comments section?

It’s a common idea that is put to good use here - most people don’t need the transcript, but it’s there if anyone does. Knowledge on demand, making it opt-in rather than forced.

If the goal were truly to avoid spoilers, then the entire post would need to be marked as such - probably by demarking it as NSFW, since many old Lemmy clients are reportedly very far behind in implementing features and that’s the only one that always works (to the annoyance of many who then cannot distinguish between their NSFW content and mere spoilers, prior to clicking).

So that’s an answer to your question, though if all you wanted was to express your displeasure at OP’s choices, I will add that you can just say that, or block them?