Because it's translated into a single machine instruction!!! "mov eax,[[ebx]]" doesn't work either - not on x86 at least, historically there were architectures that had indirect memory references :)
It's still MUCH easier to read and write. How do you even remember the order in AT&T syntax? Shouldn't the scale factor maybe be given as a shift count since that's how the hardware works? By using familiar arithmetic expressions, it becomes perfectly clear what is meant, and anyone who actually writes assembly will quickly learn what forms are allowed. And if you are only reading the code, it doesn't matter.
Of course, AT&T syntax comes from UNIX, where the zeroth commandment is "Thou shalt have no other programming languages but C and shell scripts". People who have invested the effort to memorize something like 42 different levels of operator precedence, and fluently read and write symbol vomit like "(((void *)(int [])fn(foo,bar=baz==quux++))" are obviously desperate to make assembly look EVEN MORE complicated...