Why are skeletal muscle fibers multinucleated

It is because large skeletal muscle fibers form by the fusion of thousands of individual muscle cells

Explanation:

During the developments of muscle fibers, multinucleated arrangment is formed and it is also known as syncytium (syncytium is defined as multiple cell fusions of uninuclear cells).

Multinucleated arrangment of muscle fibers makes the muscles efficient, because many nuclei can control the contractions of long cell better.