The fact that you're leaving could very well be relevant. idk what you do for a living but let's say you're a software developer.
If you're leaving in three months then you're not the best person to assign a six month project to. It might be prudent for your manager to retask you with writing documentation for the work that you've done and fixing bugs in features you've worked on instead of working on new stuff.
If it's not possible to discretely retask you, doing so without providing an explanation could make speculation run rampant among your teammates ("are they preparing to fire him??"), so it might be best to nip that potential speculation in the bud.
Plus, by them telling people instead of you, that kinda enables them to control the narrative. Like maybe, in theory, you could spin it so that you're a victim or something. And who knows - maybe that's happened before and their reaction is being influenced by their past experiences.
This may be somewhat country dependent, as well. In most of the United States employment is at will. In the United States if you give three months notice (which is, itself, a bit unusual) and your employer then does something you don't like you could just up and leave. Rather then risk upsetting an employee who's half out the door, already, in the US, it might be more prudent to take an employees desires more into consideration then it would be in other countries.