They are not. They vary greatly in structure and function.
Sure, of course the different multicellular eukaryotes vary, but the diversity is very low compared to what we see across all life, look at Figure 1 here for example [1].
I am looking at it from the perspective of the structure of the metabolic network, as this is my area of research. There is a huge amount of diversity in the fundamental structures of metabolic networks, types of metabolites and enzymes produced, etc. across all life, but from that perspective the differences between one multicellular eukaryote and another are slight in comparison. Multicellular eukaryotes have evolved very different behaviors, shapes, sizes, etc. from a relatively conserved biochemistry. When taken out of the context of an organism in cell culture, the cells are similar enough that their behavior under diverse experimental conditions can be predicted by nearly identical metabolic models, whereas you would see nothing of the sort with distantly related bacteria- the metabolism can be massively rearranged.
It seems very likely to me that our views are too reductionist, and the much of the key information isn't even encoded at the level of DNA as previously assumed.
The cells multicellular organisms are constructed from are all shockingly similar... "cells" are basically a solved problem for this type of life, and somewhat frozen in their functionality because major changes would disrupt the larger systems that they make up.