That tag is explicitly blacklisted - there's no need to have
drupal as part of the tag name, as this is an exclusively Drupal-oriented site.
For reference these are the regular expressions used to prevent those tags from being created:
We couldn't create that tag if we wanted to :)
You can find this discussed in detail here: Numeric version tags considered ... OK
Actually, using drupal-7 as synonym of 7 would be useful for those questions that are migrated from Stack Overflow, where drupal-7 is one of the available tags. Without drupal-7 as synonym, drupal-7 is going to be removed from those questions, and important information is removed from the question.
In order for this to be possible, a Community Coordinator should answer to Allow "drupal-N" tags to be migrated from other sites, which is what doesn't allow us to set drupal-n as synonym of n. Moderators cannot change the entries for blacklisted user input; that is why we depend from somebody else to fix this.
Anyway, when a question is migrated, there is always a link to the original question. This means that the tags used when asking the question are always visible. If a tag is missing, everybody can look at the tags used from the OP, and add the equivalent one, if needed.
Apart preserving the tags used when migrating a question, there isn't much use of that synonym tag.
Synonym tags are used not as shortcut for a tag, which is not this case, since you are proposing a longer tag as synonym of a shorter tag; synonym tags are used when there are users who use two or more tags to mean the same thing. For example, if users would keep using views-module as tag for the Views module, and four new questions would use it every time the tag is edited from question, then views-module would be a candidate for being a synonym of views. Since users cannot use drupal-7 as tag, there isn't the need to set it as synonym.