Why Do Cats Land on Their Feet? Physics Explains
https://www.scientificamerican.com/article/why-do-cats-land-on-their-feet-physics-explains/

This phenomenon occupied many scientists, including physicist James Clerk Maxwell, known for his work on electromagnetism. He conducted several experiments in which he dropped cats from various heights (including from open windows) and onto beds and tables. But it wasn’t until 1969 that the “falling cat problem” was solved. As it turned out, the cat’s body had not been considered carefully enough. It is not simply a cylindrical object that magically begins to spin. If you look closely, you can see that a cat’s upper and lower body rotate in opposite directions. Thus, conservation of angular momentum is preserved

 

Their just lucky I guess. ?

 

They have a High-Torque-Propeller mounted on their butt,
which can instantly spin in any desired direction,
and a Long-Travel-Suspension-System built-in.

Tree-Squirrels have a similar setup.
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.
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What's also interesting, is that their balance system is apparently hard-wired to perform the complicated maneuver needed to always land on their feet.
It must have evolved as part of their natural balance system when attacking their prey in the wild.

 

Cats (of all types) evolved to use trees, which involves a lot of jumping (and falling). Those that developed innate abilities to do those things safely and effectively had more offspring.

If you watch the big cats, especially leopards and the like that have very long, thick tails, you can see how they use it to control their motion when they make long jumps.

But these abilities are far from limited to cats -- almost any critter that spends a lot of time in the trees has well-developed skills when jumping.

 

That's HOW cats land on their feet.

WHY is another matter but possibly to avoid banging their heads on the ground when they land.

 

When I was a kid a fat horny male cat came chasing our female cat outside our house. I quickly grabbed a wooden stick and tried to scare him out by banging stick loudly on the floor. He was so scared that he fell from 1st floor trying to run away on a boundary wall. I felt bad for him thinking that I might have caused him injuries but after a few years when I realized that cats always land on their feet coupled with the next level shock absorbing system in their paws I was finally relieved that it was nothing more than a routine cardio for that fatty boy

 

My grandpa told me back in my young days, it's because their tail isn't strong enough, but their feet were.

 

Why or How?

Why? Well because landing on their spleen might not work out well.

How is a different thing, but suffice it to say they evolved not to land on their spleens.

 

Cats always land on their feet, because they are actually fluids.