Cats never miss an opportunity to jump into every space, no matter its size or availability status.
I don't need to iterate on how cats enter every cardboard box they see, or literally any other small space they locate. But first - why do they even do that? In short, cats simply feel safe inside. Having an enclosed space around them, like a cardboard box, gives them a sense of security - however real or not. They also view these spaces as strategic vantage points to pounce on prey from - so basically a place to cosplay big cats. So basically it's just another proof that cats make no sense at all. Their logic is not logicing. But this is why we love them so much.
My cat found herself in the most ridiculous situations because of this internal drive she has to enter literally anywhere. To do the fits and sits even if she doesn't fit and cannot sit. She once got herself stuck behind the washing machine because she absolutely insisted this nook must be explored - she just had no idea how to get herself out. At another time, she got locked inside the pots and pans cabinet, which is protected by a child lock. It was open for one second, and she still managed to get herself inside.
But all of that doesn't make her experience with classic cardboard boxes any easier. That child lock I've mentioned? It's there for a reason. The reason being a 1-year-old roaming free in the house because she's been walking since she was 10 months old. That kid sees the cat entering a cardboard box I left on the floor for her and thinks, "I'm going to get inside to cuddle the cat." My cat bolts away every single time. Raising cats and kids together is not a simple task.
But cats will always cat, and that means they'll fit, sit, and be absolutely smug about it. And honestly? It makes them even more special than they already are.