Apache: using %{DOCUMENT_ROOT} inside <Directory> section

Question

My virtualhost configuration file sites-available/site.conf contains (some lines omitted):

<VirtualHost *:443>
  <Directory /var/www/html/my_website>
    # set options
  </Directory>
</VirtualHost>

This sets the options in my_website directory. I can do the same with a Define directive:

<VirtualHost *:443>
  Define website /var/www/html/my_website
  <Directory ${website}>
    # set options
  </Directory>
</VirtualHost>

But if I try the same with an Apache server variable %{DOCUMENT_ROOT} the options are not set:

<VirtualHost *:443>
  DocumentRoot /var/www/html/my_website
  <Directory %{DOCUMENT_ROOT}>
    # set options
  </Directory>
</VirtualHost>

Why is %{DOCUMENT_ROOT} not working in this case?