[ expr ]) is a builtin function. Like all functions in bash, you pass its arguments as whitespace separated words.
As the man page for bash builtins states: “Each operator and operand must be a separate argument.”
It’s just the way bash and most other Unix shells work.
Variable assignment is different.
In bash a variable assignment has the syntax:
name=[value]. You cannot put unquoted spaces around the
= because bash would not interpret this as the assignment you intend. bash treats most lists of words as a command with parameters.
# call the command or function 'abc' with '=def' as argument abc =def # call 'def' with the variable 'abc' set to the empty string abc= def # call 'ghi' with 'abc' set to 'def' abc=def ghi # set 'abc' to 'def ghi' abc="def ghi"