Another frequent Reddit question: if Transformers don't reproduce like humans, why do they have gender at all? The answer depends on the version, but some ideas are shared.
Why most are 'he'
The classic 1980s franchise was aimed mostly at boys, so most characters got male names and voices. The first heroines appeared in 1985, and Arcee in the 1986 film, but for a long time there were very few.
Gender as personality
Because Transformers are born from sparks rather than parents, in many stories gender is part of their personality and self-expression, not biology. Some modern comics openly discuss a Transformer choosing how to identify.
Modern series
New shows are making their casts more diverse. For example, Transformers: EarthSpark features a character who uses they/them pronouns — a notable shift in the franchise's approach.
Summary
In most stories a Transformer's gender is how the character feels and identifies. Canonical 'biology' is secondary; character is what matters.
