Humanoid

Humanoids are generally bilaterally symmetrical bipeds with a naturally upright posture, two arms, two legs, one thorax, a neck and head with a brain located in it. The most recognizable features are their facial features. They usually look similar to Humans, with the best example being the Gallifreyans.

The humanoid form makes an excellent concentrated protein. (DW: Revelation of the Daleks)

The Daleks used humanoids as work machines, believing they had no other value. (DW: Destiny of the Daleks)

However, this was most likely due to their Dalek-centric view than to any other reason.

Humanoids are referred as "fleshy bipeds" by the Mantasphid Queen. (DW: The Infinite Quest)

Theories

One of the more popular theories as to the many humanoid species is that the evolution of Gallifreyans relatively early in the history of the universe affected a morphic field for the humanoid form that resonated across the universe and influenced the development of later species in a similar direction. (NA: Lucifer Rising)

Another implies that Rassilon seeded the early planets with biogenic molecules that would develop into humanoids, specifically ones which resembled Gallifreyans. (BFA: Zagreus). The truth is that creating non-humanoid aliens costs too much.