I wouldn't call Engineers or Lekgolo worms humanoid. Regarding the other species, possibly Halo assumes that humanoid is the most effective shape for developing civilization and space travel. It's also possible that they were engineered by the Precursors. As far as I know it's never explicitly said.
Without the dexterity brought on by being humanioid, a sentient species would not evolve easily. A species would have to have hands similar to ours, especially the thumb, or they would not be able to construct anything. For example, a creature with tentacles and no fingers couldn't build anything; it couldn't grab on or finely tune a structure like we can. If gorrillas were to become intelligent, they would evolve very similar to us just because their hands are pretty much exactly like ours. All of their structures would look the same as ours throughout history.
This might be a little unrelated, but intelligent alien life would be difficult to create. Humanity is very, very unique and lucky. Earth was an accident; we have never seen a planet like ours. We suspect that there are some, but we can't tell yet. Don't get me wrong, I believe in aliens, but I don't believe that they are some super-advanced war-loving species. Originally, Earth was a molten ball of volcanos and hot rock. Scientists believe that we were hit by a moon-sized object early on in the planet's formation (about 4 billion years ago), which altered our rotation around the Sun and corrected our path. This luckily allowed the planet to cool and begin to create life. Also, Earth had a heavily carbon dioxide atmosphere at this time, which allowed for the formation of plants; the plants sucked in the CO2 and allowed for oxygen to take its place. The oxygen, even though in small amounts, allows us to live today. But that's not it. Earth has a molten iron core, which creates a magnetic field around the planet. Were it not for this hypothetical shield, the Sun would boil us alive with radiation and UV-rays.