Why are Europeans dinner tables rectangular while Chinese dinner tables are round?
Why are Europeans depicted eating at a rectangular table and not a round one? Were other types of tables used?
Where did you get this idea ? I've never heard that. And I've never got the impression that round tables weren't common in Europe.
Why do you think that there is a trend behind table shapes in these two cultures?
A very common and traditional type of Chinese dinner table called "Eight Immortals Table" that's square and seats two on each side. Ancient depictions also shows mostly rectangular tables.