Why doesn't muscle strength depend on its length?
It's simply that the fibers are arranged end-to-end. If you simplify this down and imagine a single muscle fiber you have actin and myosin like this:
ligament-|actin-myosin-actin-myosin-actin-myosin

If you think of this as a strand of cotton pulling on a weight (ligament). Each group only pulls along the length, so the strength is not multiplied, but rather related to the strength of the individual fibre.
In reality muscles are stacked into bundles, which are much stronger as all the fibers can contract pulling with the strength of the multiple ends. Like this:
         |actin-myosin
         |actin-myosin
ligament-|actin-myosin
         |actin-myosin
         |actin-myosin

This is like the multiple strands of cotton making up a rope, the strength comes from having lots of them distributing the load.
This is something called physiological cross-sectional area, which describes the strength of a muscle by estimating the cross-section of the muscle across its thickest part.