Because it is not good for tangling.
For example, tangle the bellowing src block, the tangled source code file will has `(ref:hello)` too.
```
#+begin_src emacs-lisp :tangle "data/code/src-coderef.el"
(message "hello, world") (ref:hello)
#+end_src
```
The tangled source code file looks like this:
```
(message "hello, world") (ref:hello)
```
This absolutely is not a good idea.
And after a test, I found I can't tangle the bellowing src block:
```
#+begin_src ruby :dir "data/code/src-coderef.rb"
puts "hello, world" (ref:hello)
#+end_src
```
Even with a comment char prefix:
```
#+begin_src ruby :dir "data/code/src-coderef.rb"
puts "hello, world" # (ref:hello)
#+end_src
```