I feel like they forgot to include a radio message from Atlas at that point, 'cause there is a reason you have to do it- to get the door open. However there's no way for your guy to KNOW that, unless he's aware he's in a video game- "Locked door? Hmm, I'll be the only way to open it will be something conspicuous and nearby!"
It definitely would have made more sense if Atlas had popped in with "Hey guy! That door's busted, but if you inject that giant container of fluid into your wrist you'll gain freaky superpowers that'll let you unlock it! What's that? That much liquid would cause an insane amount of pain and swelling and probably burst your veins? Nonsense! Get splicing! No, wait. Do it slowly so I can listen. That's nice."