Because by the time of writing, Kubo had not designed what shikai and bankai were! He had no concepts of it. If you remember, Renji did actually release twice the first time he fought Ichigo, which is virtually impossible now, since a shinkai can only be released once. Furthermore, Renji explains himself that the change of his sword is the true shape of his spiritual power, and it is likely Kubo was aiming for that when he designed Ichigo's shikai. Therefore, at first, Kubo probably had an idea that it would be cool if sihinigami could change the shape of their sword to match their personalities in combat, but that it would be named shikai came much later.
It is also most likely a design option, in that Kubo wanted Ichigo to have a new sword and it would be boring if he could revert it to normal. Furthermore, it is very explainable that Ichigo can't revert it to an unreleased state considering that he, back then, could not control his reiatsu very well. It is also likely that Ichigo was so strong, even for a shinigami, that it was not possible for him to revert back even if he could. Whereas for other shinigami, learning the name of their zanpakuto can take many hundred years, but for Ichigo, it took three days. Since the other zanpakuto he acquired that was without a name was not explicitly his, he may just have discarded an unreleased state for the sake of fast evolution.
There are many ways to see it, and I find none illogical. Kubo has also addressed the issue himself with Unohana stating that his sword may just be one of those very few, rare swords that are in a constant shikai state. Just because Ichigo and Isshin are related, it does not mean that ichigo must follow a similar path to Isshin and vice versa. There are fundamental differences between them, such as it is likely Isshin was taught to be a shinigami at the Shinigami Academy and that Isshin never skipped several parts of shinigami evolution.