Frankly, because it's a one-liner working on one string. The performance difference isn't going to make up for typing extra parentheses, though it is something that people should be aware of. I could have left out more parens, but I thought it would make a hard-to-read answer.
perl -pe 's/./sprintf"&#%03d;",ord$&/ge'
Caution: Contents may have been coded under pressure.