They do not lack motivation. You may not like their motivations, but there they are. Curses are, by definition, motivated by their existence. We had plenty of flashbacks for each character (In a trope-reversal, usually when they are close to death) and what motivates them.
Sukuna, on the other hand, does not need motivation. He's at the summit. The rest is "pulled" towards him. So He's reactionary rather than anything else. You could argue that resurrection is his goal but I wouldn't say that.
I feel it's rushing if anything else
I didn't say they lack motivation. I said they lack motivations that
resonate with the audience (aka the reader). That introvert kid that Mahito turned into a sorceror, then corrupted and Yuji failed to rehabilitate was the (only?) attempt in this whole title by the author to write a character that the readers can relate to.
Sukuna's motivation: "I am the king of the jungle. I eat, sleep, kill, and basically do whatever I want, whenever I want, and don't really understand connection to others because I need no connection to others, being completetly self-sufficient." I get that readers might be attracted to that in a sense of wish fulfillment.
Gojo's motivation: "I'm so awesome that once I hit age 18 I basically became bored, phoning it in all the time and mostly living for the lulz. I'm kinda lonely too, it'd be nice to find someone I could relate to." Again - how is that relatable to ANYONE?
Yuji's motivation: "I'm a normal guy, wracked with unimaginable guilt as though I try to do good in this world, I keep failing and being at least partially responsible for mass murder. oh and because of me the world's going to end". Again HOW THE HELL IS THAT RELATABLE?
Maybe, just maybe, for a few chapters we felt a little bit like we could relate to Fushigoro Megumi's motivation to save his sister in this culling games. You know, the sister that had next to zero on-screen time, zero interactions or character development. Who was "fridged" from the start of the story, brought out of the fridge to serve as a reason to fight, and then immediately thrown in the dumpster permanently in order to create a pathos and grudge.
Name one character whose motivation is relatable in this series. The whole "everyone here is nuts and their craziness only grows with their cursed energy" thing created this idea of gameifying mental illness by turning it into a source of magical strength, but kind of abandoned that idea and just left everyone with a "unrelatable" motivation.
Incidentally,
motivation can truly make or break a series. Bleach, for example, peaked in readership with the Soul Society Arc. You know, the one where young people are literally fighting against society's rules to save the life of their friend. It then tanked when that fight became all the characters you hated until now are on the same team and they're fighting motivationless flunkies until they faced the big bad who was...literally a near-god-like character whose big master plan was to become...more god-like. Then later on Bleach became a fight against MAGIC NAZIs, and again just a pissing match between god-like powers with no greater conflict than an old grudge. And everyone stopped bothering to read and it was canceled.