Thank you so much for all the answers, they have all be so helpful. 🙂
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You have to think about us (and all other matter) as not moving separately through space and through time, but rather moving through *spacetime*.
On a ferris wheel, you are always moving at the same speed, right? However, the faster you are moving in the vertical direction, the slower you are moving in the horizontal direction, and vice versa. Your *speed*, the magnitude of your velocity, remains constant, rather it is the weighting of the horizontal and vertical components that make up that speed that changes with time.
Spacetime works the same way: Imagine a 4 dimensional analogue of velocity that tracks how “quickly” you move through spacetime. This quantity (rather unimaginatively named “4-velocity”) is constant for all matter. In the same way as the ferris wheel, that means the faster you move through space, the slower you move through time. The magnitude of your 4-velocity remains fixed; it is only the weighting of the spatial components vs the time component that changes.
Humans are, relative to each other, effectively stationary: Nearly all of our motion through spacetime is, from any of our reference frames, through time. This is why we can, in daily life, treat space and time as unrelated quantities. u/DoctorKokktor’s answer is a great example of how that breaks down in more extreme environments. If you ask why the universe behaves this way, we could point to the fundamental fact that the speed of light appears to be the same in every frame of reference, from which all the rest of this is derived. As for why that’s the case, in physics the answer to “why” questions is always eventually “that’s just how the universe seems to work”.
One thing physics teaches you is that our brains evolved over millions of years to keep us alive on a cold, fairly small, low energy rock where nothing is moving very fast. The universe in more extreme environments is under no obligation to make sense to our extremely limited intuition
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