There are a few reasons for it:

• In countries at peace handing over these kinds of armaments comes under public scrutiny - and public announcements from one country shift public mood in others.

• Russia would probably notice at least some of the tanks being moved anyway - they require a lot of logistical support.

• Since Russia can't be invaded for the war to end unfavourably either their morale or ability to fight needs to be crushed. Announcing a shitload of weapons hurts the former, the weapons themselves hurt the latter.

There's also not all that much Russia can do that they wouldn't attempt anyway. For instance, if they were capable of an offensive to pre-empt these deliveries they'd probably launch one anyway even without knowledge of them.

Also, Ukraine and the West have been quite good at misleading the Russians so far this war - for example they were led to believe that a Kherson offensive was imminent, and then were hit with an entirely different offensive at Kharkiv. So there is a good chance a vital piece of the story hasn't been made public.