This is how Apple always announces WWDC. Last year's announcement was on March 14th.

https://www.apple.com/newsroom/2019/03/apple-to-host-annual-...

The announcement is really just for confirming exact dates. Everyone thinking about attending knows that it's in early June and isn't being surprised.

It's strange but I've noticed most conferences are planned in advance.

the announcement is about the conference being virtual. WWDC is in June every year.