Indeed, managers are somewhat involved in bringing/offloading players IRL but this is a qualitative conversation with the business team (for example, "how much I want that player on a scale: from indispensable, quite a lot, somewhat, a little, to don't bother...). FM allowing managers to arrange contract details is *not* realistic, but it's fun. (And you don't want to delegate it to AI staff to make stupid decisions anyways...)
Likewise, managers shouldn't select jersey kits before matches, but the game often mixes up colors, and, two, it could be fun.
As I said before, FM allows managers to make decisions on many topics in ways that do not follow real football practice, but these decisions are lots of fun anyways. Selecting jersey kits would be just one more possibility.
But, hey, I'm not losing my sleep over this topic...