Why doesn't Dora have eyebrows?

This isnt really an answer.

I noticed after reading your question, that indeed Dora doesnt have eyebrows.
After looking at a picture of her, I’ve come to the conclusion, that she probably doesnt have any because of her bangs. They do hang pretty low on her forehead.

That’s certainly true. However, why don’t we catch a glance of them when she’s doing a super-soccer kick, walking in strong winds, or participating in other activities?

Boots doesn’t have eyebrows, either. I was going to say that only (what would otherwise be) inanimate objects have eyebrows, but Swiper has eyebrows. This doesn’t answer your question, but now I am perplexed.

I forgot that Boots doesn’t have eyebrows either. It just seems odd because they are the two main characters.

Well, that’s a question that only the cartoonist can answer.

I have only a rudimentary knowledge of Dora, but just guessing by the examples you cited in your question it seems that the artists are trying to give emotions to inanimate objects. (trains, stars, gates) Emotions in animation are very often represented in eyebrow form. Dora is human and therefore needs no assistance.

I checked, and Dora’s father, mother, grandma, cousin Diego, etc. all have eyebrows, which makes it even more odd that she is the only person who doesn’t.

It takes a lot animate that stuff, and when you must do it in every second of every episode it gets long and hard/boring.
If you haven’t noticed, the creatures that only appear for a short time-period always have eyebrows. Why? Because it’s not such a struggle to animate eyebrows for 30 seconds rather than 30 minutes.