There are a few possible explanations for this, but of course it is impossible to know for sure.
1. They could have evolved in an environment that has similar influencing characteristics to ours.
2. They (or some of them) are our future selves.
3. They (or some of them) are from another dimension or exist in a way that we wouldn’t recognize as life, and choose their forms when they interact with earth.
4. Ancient aliens aided in our creation/ modification.
i have a theory that all alien/ET intelligent life is some form of human and we all started out from the same type of cell but have developed differently due to environmental factors around the universe thats why all intelligent aliens such as the Greys or the Nordic, or even the reptile have a similar form to our own (bipedal, 2 arms, 2 legs, an intelligent brain, etc).
I like it! It is very similar to panspermia, which I think is my favourite explanation.