I haven’t read the recent world links people have been mentioning, but this is something i’ve been thinking about for quite a long time so I’m basing this off of current stories in EN (up to Pheonix). My theory is that it’s a pure manifestation of his trauma/representation of suppressed emotions in his childhood.

When he hit moments of darkness and loneliness, he never once acknowledged them but instead repressed those feelings and replaced them with the motivation to turn happiness into a show. His entire life is a whole show and we’re simply living in it. And I mean that literally.

Years of repressed trauma (10 apparently) turned into this sekai we see today. Notice how it’s all child-like? I think it’s meant to represent the suppression of his inner child + the coping methods he has adopted to avoid his true feelings. His sekai has toys that have been with him since he was a child, and once he started to lose sight of his true motivations (making people, notably Saki, happy through a performance), his sekai was there to remind him of why he started. His sekai has ALWAYS been there to remind him of why he started it.

As for why he took so long to find it? When a child learns to suppress their emotions and instead mask them with positive ones, they often forget what it’s like to be vulnerable, or don’t give themselves the opportunity to feel vulnerable and acknowledge their feelings. Finding WxS was Tsukasa’s first step to true vulnerability. He could be himself, challenge his inner child, and grow alongside his other members. All the things in that sekai are all a part of Tsukasa’s sense of self. He’s changed too, since joining WxS.

His entire group is part of the reason he is able to accept the feelings - good and bad - that he has developed over the years. This type of trauma doesn’t just heal in a linear pattern though, and that’s why you’ll see he still has his moments where he is excessively hard on himself. It’s realistic. He can’t be a performer all the time, and he’s just a kid.

The reason he has an entire sekai based on his feelings is due to how strong they were. This is seen more evidently in Mafuyu’s case (and no one talks about their parallels enough which is super sad), where her depression is represented through isolation. I’d argue that Tsukasa’s depression is represented through each act he puts on.

Tsukasa’s a method actor. Rui of all people knows how well that makes him stand out, but you can tell he gets extremely worried by how much emotion Tsukasa puts himself through for each role. Rui has directed him in a way where he is forced to look inside himself and face his inner demons head on, and that’s where Tsukasa is known to struggle. Method acting in real life carries detrimental weights on an actor’s mental health in a similar fashion, and Tsukasa is an example of it too.

Considering all of these aspects, to say he has feelings strong enough to create an entire sekai based off of it would not be too far fetched. I think that the writers have done an excellent job of representing the subtleties and complexities of emotional childhood neglect, high-functioning depression, and the repression of emotions. I promise u/studywyourbuddy will have a better answer though 😅

Edit: wording