They leak online documents because its usually Gaijin’s braindead mods rambling about superior russian technology that gets implemented as invincible in game lol.
Also pretty much all these leaks are considered sensitive info, not top secret. It’s not common on the internet, but really easy for many people in the military to access because its usually basic things like a flight manual.
Not to mention its also mostly retired or previous generation tech, so this material is probably coming from off service members anyway.
Hell even the China MBT “leak” wasn’t illegal because it was already declassified.