Why is a pseudocoelom considered to be a false coelom?

A pseudocoelom is considered a "false" coelom because it is not entirely lined by mesodermal tissue, unlike a true coelom. In organisms with a pseudocoelom, the body cavity is situated between the mesoderm and the endoderm, resulting in a cavity that may not provide the same functional separation and organ development as a true coelom. This distinction affects the structural organization and complexity of the organism, typically seen in groups like nematodes.