//spoilers
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After Sanyu opened the blindfold, og!Sihyeon became hysterical, saying, "don't look at me, don't look" because the least he wanted was for the members to find him in that state. so Sanyu had to calm him down (this explained why even in recent chapter Sanyu still said, 'I didn't look) and took og!Sihyeon home. But even at the dorms, og!Sihyeon still got scolded by Uihyeon who knew nothing about his mental state, so yeah long story short Sihyeon drove alone after the whole shitshow because he still had work that evening and that led to the accident. Considering all that, I understand why Sanyu was intent on keeping it a secret. He didn't want a repeat of all that, especially about the part where he almost lost Sihyeon (although he really did lose og!Sihyeon).

Thanks to the spoilers, I get why Sanyu didn't want to tell Sihyeon the truth of what happened that night