Because it's not as if convergent evolution exists or anything. Sometimes it feels as though the extreme proponents of starfish alien design often have a wanting understanding of evolutionary biology (and the tendency toward carcinization). It's surprising how uncommon it is for starfish alien designs to have crab elements.

(Note: I'm using the TV Tropes definition of "starfish alien" here).

Okay but like, the human bauplan is extremely rare in nature, and requires a lot of extremely specific steps to reach. It's not likely to appear very often, and isn't an instant guarantee of or requirement for sapience. Not to mention for the longest time people just assumed intelligent life would be humanoid because that's the only form of sapience we're familiar with. People who advocate for starfish aliens are trying to make the point that alien life doesn't need to look exactly like something we're familiar with to function. Convergent evolution isn't some magic bullet that will make everything evolve to look the exact same.