[ilovepeppapig]

Try pressing your hand against a flat surface and then slowly applying enough force in the way parallel to the wall so the hand starts to slide. After some practice you will feel the point of losing traction. The concept is the same with tyres. F1 as of 2018 actually doesn't allow drivers to push 100% all the time, otherwise the tyres will overheat and the strategy is screwed.

Plus, 4 tyres may have different traction depending on car balance (braking, accelerating or cornering), downforce (nearly zero at 50km/h but at 300km/h F1 car is on rails, and different for the front and the rear), track specifics (for example, most clock-wise tracks put a lot of pressure on the front left tyre) and many little other things. It's really complicated.