I’m wondering if it’s pseudo religious with humans being created in God’s image (Ancient Aliens stuff), the human-centric idea that intelligent life must resemble us, it being easy to make costumes for movies and TV when all you need to do is paint someone’s skin, or if SciFi writers were going for the uncanny valley effect for example.
Like others have said, budgetary restrictions and limited imagination is the honest answer, but there are a couple somewhat interesting “handwavey” explanations a sci-fi universe can use. Off the top of my head:
Directed Panspermia: Life on Earth (and other planets) was seeded by an intelligent higher order. Evolution occurred mostly naturally, but all following the same starting blueprints with variations to account for environmental differences. Eventually this leads to intelligent, humanoid life. Basically, sci-fi intelligent design theory.
Convergent Evolution: Two unrelated organisms can evolve similar traits because they happen to be best suited to certain tasks. For example, the number of times crabs have evolved independently, because crab-like traits work universally well for their shared niche. So in theory, if humans are distinctly advantaged for our evolutionary niche, it stands to reason that other forms of intelligent life might resemble humanoids as well.