Great question! But the deeper question is: Why haven't you? 😊
Ghost is a completely open source product, which means that other than a very few full-time salaried employees of the non-profit organisation which steward it – it's built almost entirely by a small group of volunteers.
Now, it's impossible for our little core team and small group of volunteers to work on every single thing - and, of course, they have their own ideas about what to build next, too!
With a closed platform, you have no recourse if the core team isn't focused on your pet feature. The conversation ends there. The entire point of open source is that if you're knowledgeable and passionate about something which would improve the software, then you are still able to implement it yourself.
That is the freedom which open source grants you.
If you choose to send your implementation upstream in the form of a Pull Request, and it is accepted as a good approach which will work for all users and not just you, then it can be accepted for others to also benefit from your work.
If you don't have the ability or the inclination to build a feature yourself, then you can always pay someone else to build it for you – commonly known as sponsoring an issue. Either you have a motivation to build it yourself, or you provide someone else with an economic motivation to build it for you.
Not every feature makes it upstream, though. It may be that you build something custom just for yourself. That's ok too. For example, Apple use Ghost internally to run hundreds of micro-sites for their different teams, but their own security policies meant that they elected to fork and remove Ghost's user system and replace it with a SAML implementation that was compatible with their corporate auth. Totally fine. Not something which will ever make it upstream. But the fact that Ghost is open source meant they were able to do it.
Most features in Ghost start out with someone wondering why something doesn't exist yet, and then deciding to build it themselves.
If you're interested in contributing to Ghost, here's a guide to help get started.