In the physical world, sure, but there's no reason why a virtual linear slide could not be made endless

You'd have a discontinuity. A knob naturally rotates back to its start. A slider never does and would have to pacman-wrap itself back to the start.

There's no motion you could make that would infinitely increase without a break. A knob you can just move your cursor in a clockwise circle infinitely.

Yeah but that doesn't have much value as you lose the value indicator.

If you don't need a value indicator, you don't need a circular knob as an affordance. You can have a whatever as it just reacts to your input.

If you do have a value indicator which is "infinite", such as a numerical value display, it's better to make it interactive and place the interaction on top of it, instead of splitting the UI between a value indicator and the input.

A lot of software does this.