Because Leave Open votes are a mechanism that works exclusively within the review queue.
The core mechanism for community-driven question closures is that
a quorum of five 3k+ regular users agree that it should be closed.
This is a hard-and-fast rule, with the only exceptions being (1) moderators, who have full control to be used where appropriate but in as minimal a set of circumstances as possible, (2) the dupehammer ability of gold-tag-badge holders, and (3) the ageing away of close votes (one a day after four or fourteen days, depending on views). Messing with this rule is a major design change to the system behaviour, and it should only be done after substantial analysis and for reasons of extreme importance. (Or, to put it bluntly: that's just not going to happen. SE is busy enough with other aspects of the system that the type of engine redesign you're proposing is simply not going to end up on their to-do list.)
The other side of the closure mechanism is the review queues, which exist to make sure that off-topic questions get enough close-capable eyeballs to ensure that they're closed if they need to be. To be clear:
the primary purpose of the Close Votes review queue is to prevent a situation where an off-topic question gets one closure vote, very few views, and then it slips off the bottom end of the front page, never to be seen (or close-voted) again.
It works by making sure that any such question ends up in a review queue where, if it does merit closure, it does get closure. However:
the Close Votes review queue is not intended to act as a Death Row corridor where the only way for questions to get out is by getting closed,
which would effectively mean that a single close vote would doom any question, negating the core rule above. There thus needs to be a mechanism by which on-topic questions cease to be considered, and this is what the Leave Open vote does: if the question gets enough Leave Open votes (at present, three of them), it gets removed from the queue. This is the only function that the Leave Open option was designed to achieve. (Moreover, given that the Leave Open option is only accessible within the review queue, it does not make any sense to have it act outside of the review queue.)
And this, of course, naturally raises some questions:
So why was it designed this way?
Because the Close Votes review queue was designed quite a while after the regular closure mechanism was in place (meaning: all the way back in 2012, i.e. four years after Stack Overflow started, with close-voting built from the ground up at the very beginning of the Q&A engine design). Since the review queues were an addition to the existing mechanisms, they were not built to modify the existing behaviour.
OK, but why can't it be changed now?
Because the core Q&A engine needs to serve the needs of an enormous variety of environments: from the voracious high-traffic environment of Stack Overflow, through a wide cast of 100+ Stack Exchange sites, each with its own community and culture, to the smallest beta sites with low traffic and relying on moderator actions to keep the flow in shape (or trying to pass that baton over to community moderation). This is a system used by millions of people weekly, and changes to the core behaviour should only be done if the disruption to the existing processes can be justified by the extreme need for the behaviour change being considered.
And here there isn't any such need: the system is working just fine as-is.
Your proposed re-design could indeed work, but there's nothing in your post that even hints at an argument that the current system doesn't work. Both systems have their own logic, each with its idiosyncrasies (in the existing system, Leave Open only acts within the review queue, which some people (e.g. you) find confusing; in your proposed re-design, there is a review-queue-only option which significantly alters the balance outside of it), and, frankly, the logic gaps in your system are worse than the existing one.
