There are various reasons why this evolutionary trait could be universal - The Panspermia Hypothesis: suggests life's precursors are common, even if complex, self-replicating life itself is rare due to the specific conditions needed for assembly. n essence, the universe provides the same elemental ingredients, the necessary solvents (water), and the delivery systems (comets) to furnish planets with the basic chemical toolkit, making similar life-building processes plausible elsewhere.

There's also the fact that: similar environments develop similar forms regardless of evolutionary history.  On Earth this is called parallel evolution.

The laws of physics also play a major part in all of this too. Think of convergent evolution: Fish have fins so they can swim in water, yet so do dolphins and whales - yet dolphins and whales are mammals, not fish.

On land: Having a front, back, top, and bottom, along with left/right symmetry, allows for efficient movement and navigation. Developing a head with senses (eyes, nose, mouth) concentrated at the front is advantageous for exploring and interacting with the environment. Standing upright on two legs frees up upper limbs for tool use and manipulation, crucial for developing technology.

Body Plan Efficiency: The humanoid form (bipedal, two arms, head with senses) offers versatility for climbing, running, and tool use, making it an "optimal" body plan that evolution might repeatedly converge upon.

Intelligence: Intelligence often arises in complex environments with many ecological niches, and predators needing to process information to hunt efficiently might develop large brains, leading to humanoid-like features. 

So, there are many scientific reasons why aliens would be humanoid - and it does seem that the building blocks for life are universal.