Who is to say that is a universal phenomenon? May have something to do with our level of gravity, life cycle of the blastocyst, metabolic requirements from respiration, other fauna competition, etc. Which is not to say there is a better alternative on the table, but I would hesitate to say that we should expect crabs on other planets.

The fact that there are a large number of crab-like land and water dwelling organisms, and in a range of sizes and regions, suggests there's a pretty big range of environments that crabs thrive in. That's still far from universal, but it's also not very limited either.

Earth has been through similar changes too. Life started in water, a "low gravity" environment, then moved onto land. Similarly, the atmosphere was initially very oxygen saturated, causing animals to be very big. Declining oxygen rates in the atmosphere caused animals to converge to a smaller size. But, we don't see extreme differences when you compare animals from millions of years ago to today despite that the ecological parameters have changed significantly.

But those are all coming from the same progenitor life form. Initially selected components are relatively fixed from that point onwards. For example, mitochondria having their own genome and replication machinery feels like a poor solution, but it is "technical debt" that can never be corrected. A different set of circumstances could have favored all organisms having skeletons instead of exoskeletons, a singular orifice for ingestion and excretion, etc.

The existence even of mitochondria is insane though. A cell happened to eat another cell and instead of just digesting it, the prey cell fused with the predator and became mitochondria. Bananas. Similar story with plant cells.

So, monkeys had to type for only a half a million years after all.