Why does a space in a variable assignment give an error in Bash?

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Answer 1

It's not a convention in bash (or, more generally, POSIX-family shells).

As for "why", that's because the various ways of doing it wrong all have valid meanings as commands. If you made NUM2 = 4 an assignment, then you couldn't pass = as a literal argument without quoting it. Consequently, any such change would be backwards-incompatible, rather than being placed in undefined space (where extensions to the POSIX sh standard need to live to avoid constituting violations of that standard).

NUM2= 4 # runs "4" as a command, with the environment variable NUM2 set to an empty string
NUM2 =4 # runs "NUM2" as a command, with "=4" as its argument
NUM2 = 4 # runs "NUM2" as a command, with "=" as its first argument, and "4" as another