the ones that are there have been determined to be necessary by someone (or someone actually paid for them to be present).
nobody has paid for "these few" to be included AND you overlooked that "these few" are not likely "few" completely. I strongly suspect that the admin dropdowns for statuses would become unusable if ALL statuses possible were included. Having to scroll through several pages of dropdown to change a status is not a viable alternative to a few special users having to add the status they personally want.
Considering how easy it is to create a status, that's my opinion. But I don't happen to be responsible for the codebase: When PoundTeam installs a server, we routinely customize the server in our special way as we see fit for most clients to use. Such as adding 50 phones, the DGG firewall, locking down some folders. If you want the system custom tailored for You, you'll have to do it. If you want it custom-tailed for ... everyone, that's impossible and more importantly it's cumbersome to prove that it's impossible.
If you disagree, consider adding a Vicidial Issue Tracker which includes the statuses you'd like to have added (and why). Who knows: You may get your wish.
But the only ways to be Sure is: Pay the Vicidial Group or Do it Yourself during the buildout.