graves are for the living and seto kaiba’s crashout was valid. explanation below.
thinking about the materiality of mourning and how actually tragic the months post ceremonial!duel/dsod-era would have been for kaiba and how in this context his actions start to make a bit more sense
granted, i do not think it would have made altogether much of difference because kaiba finna kaiba but remember atem was a spirit, he left no body, there was no grave to visit, no shrine/headstone to burn incense at or lay flowers on. there was no resting place for kaiba to mourn at (well kinda, there was the tomb of the ceremonial duel but he literally had to dig that shit up himself).
graves are for the living not the dead. a place to remind us of our loved ones, solemnly reflect on our relationship with them and death in general. it serves as a meeting point where the living can feel like they can connect with those departed, and all those who remain can come together to share their mutual grief and mourn collectively with one another’s support.
atem has no grave where kaiba might visit and accidentally meet yugi or anzu and therein get the support of a mutual victim of loss, no place he could sit and ‘talk’ to atem (not that kaiba would take it or do that but he simply doesn’t have the option)
kaiba has nothing and more than that per the manga timeline which dsod follows he didn’t even get the chance to attend the ‘funeral’ and make the last tearful goodbyes of the yu-gang. the yu-gang even without a place to mourn at least got some kind of closure. the knowledge that even if it made them sad, atem was making this decision and doing what he wished (was he???)
kaiba. got. nothing.
with this context in mind, it makes sense he would create the AI-tem, just for the chance to talk (duel) to his beloved. it is why he is so fixated on and belligerent about the puzzle. in the Japanese version of dsod he talks about burying atem in his unfulfilled consciousness, but that’s because there was no where else to do so, no external place or object to direct his grief, except of course the puzzle. it’s the only thing that remains which he is aware of that connects to atem. it’s why he holds it with such reverence and speaks to it like we might a departed loved one. he genuinely believes it contains/may be conduit for atem’s spirit and wishes to protect it as such. because it is all that remains of basically his only friend.
so even when shown that it no longer actively contains atem’s spirit (causing visible distress) he is in denial because what then? where is his loved one? how else can he reach him? where can he go to attain some semblance of closure?
as i said earlier, would having a grave have stopped the kaibanopticon? probably not, kaiba just be like that but the reality is people need to externalise their grief through objects like graves else it ends up festering inside of us and resulting in space elevators and breaking into the duat to attain some kind of closure (at least if you’re kaiba)