Evolutionary biologist L. A. Borradaile once defined carcinisation as, "one of the many attempts of Nature to evolve a crab."
According to a 2017 paper from the Biological Journal of the Linnean Society, this evolutionary process has occurred on at least 5 separate occasions in the last century alone:
Curiously, not only did the crab-like habitus evolve independently from the 'true' crabs (Brachyura), it also evolved three times independently within anomurans. […] Although enormous morphological disparity is observed in the internal anatomy of the crab-like taxa, reflecting the fact that the evolution of the crab-like habitus was indeed convergent, various corresponding dependences are found across the different lineages between the external characters of a crab-like habitus/morphotype and inner structures. In other words, as a result of carcinization certain structural coherences led to the specific internal anatomical patterns found in crab-like forms.
"Convergence" in this case describes independent evolutions that nonetheless result in analogous structures. Which is to say that nature just really likes the crab shape ("the repeated formation of a 'general habit of body of a crab' in the evolutionary lineages of Anomura which resemble 'true' crabs," as that paper puts it) for some inexplicable reason.
This leads to only one rational conclusion:
And this, in turn, reminds me of one of my favorite clips from the very brilliant work of historical fiction known as Clone High USA:
One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea) [Jonas Keiler, Christian S. Wirkner, Stefan Richter / Biological Journal of the Linnean Society]
Image via Wikimedia Commons (CC 4.0)