Semantic versioning works only for libraries and small projects. Bigger projects simply cannot keep the strict scheme up, because they whenever they need to make bigger changes, a certain instability cannot be avoided. Either you adopt a major versions only release scheme (like firefox or chrome) or you block yourself due to the formalities until most contributors give up.
Never forget that TYPO3 is an open source product that is driven by volunteer developers. You need to keep their motivation up in order to keep the contributions coming. Keeping the motivation to contribute high, is one of the key factors for keeping an open source project alive.
We do not have the manpower to incubate bigger changes in a way that they can be merged at the beginning of the development of an LTS version. Because of this, we need to develop and merge changes as they come in. For bigger changes, that means that some minor versions might be a bit more unstable than others. We do try to develop TYPO3 as stable as possible, but certain changes cannot be avoided if we want to keep the pace up. And we need to keep the pace up, otherwise maintenance becomes harder and we cannot adopt new technologies or use synergy with other open source projects, because our code base is too ugly and technical problematic to be combined with modern concepts.
As Jigal already explained very well, the current concept was derived from many discussions and after trying different concepts and is a compromise of the needs of all involved parties.
Each minor version is centered around a certain topic that should get tackled during its development time. The numbers depend a bit on the schedule that is defined by the predefined release dates and the events around it. So we will properly have between 6-8 minor versions. The last minor version then becomes the new LTS version.
The major version -1 is coincidental in the last two releases: 4.5, 6.2, 7.6, 8.7