OK, so I wasn't seeing things... Keyboards that do have varying keystroke force do indeed exist. I googled a lot and eventually found one such keyboard (I didn't check if it was a mechanical switch or not).
Here's part of the text:
... keyboard that has three different built-in keystroke forces, 70 grams, 60 grams and 50 grams allowing professional gamers to intuitionally make accurate commands with these different keystroke forces.
The Gigabyte Aivia K8100 Gaming Keyboard also features mainstream gaming control buttons such as WASD, Ctrl, Space and arrow keys, which are most commonly used gaming keys are assigned 70g weight for elastic force.
The other keys commonly pressed by the little finger such as Tab, Caps-Lock and Shift buttons are assigned 50 grams to reduce pressure on the finger.
Now I'm 100% sure I remember seeing a mechanical keyboard's "pressure map" that showed which key had how many "gram".
I don't remember which keyboard it was but it makes a lot of sense: my pinky's would be happier if the keys I'm hitting with them were a little bit lighter than the other ones (I love my Model M, but I'd still like the keys to be easier on my pinkys).
If I bolt mod one of my M, can I someone do a mod that would make a few keys lighter?