In Issues for Drupal core, different versions of Drupal were mentioned. If Drupal 8 is obviously better than Drupal 7 and prior versions, why do the issues for Drupal 7 exist?
the version 8 of drupal exists which is obviously better than drupal 7
Yep Drupal 8 is arguably better, but...so what? When a car manufacturer releases a new version of a model, does everyone throw out the old model immediately and just get a new one? Of course not; if they can, and wish, to upgrade to the latest version, they'll do so in their own time.
In the meantime it would be ridiculous for mechanics to stop fixing the older model, right?
Same story here.
Below is an attempt to give an answer to this part of your question:
why do the issues for Drupal 7 exist?
Line 17 of this
.make file looks like so:
projects[features][patch] = "https://www.drupal.org/files/issues/ignore_hidden_modules-2479803-1.patch"
In other words, to install Commerce Kickstart, this is one of the patches that needs to be applied also. In this specific case the patch appears to be related to the Features module, as detailed/explained in the issue titled Ignore hidden modules when mapping components. ... a D7 issue, did you notice?!?! More specifically, comment # 1 of that very issue is where that patch (as referred to in the make file) was posted.
So with that, its because that D7 issue (still) exists, that it is (still) possible to install Commerce Kickstart. This is just 1 specific example, but there are zillions of patches that exist throughout drupal.org. And if you really think about it, consider the issue queue as some sort of online storage to not only attach such patches to, but also to get an idea (aka documentation) about what a specific patch is about.
But wait, there is even more: your question is only about D7, the beauty of all this is that it applies to any Drupal version, starting as of Drupal 4.x, and including 5.x and 6.x also. So to continue the "car manufacturer metaphor from a previous example: the issue queue is like the (very only!) warehouse to find the spare parts for the old models also ... including the instructions/documentation about them.
PS: the patch example I used here is about a contributed module. Though there are similar patches that are related to Drupal core. Such as what is described in the question about "Patch a module loaded by a distribution".