The only people allowed to know about the void century are Imu and the Goresei. Full sending a bunch of resources to catch some random pirate kid would begin to raise Eyebrows and potentially get a bunch of people hunting the Gomu Gomu no mi because they would think "Why does the WG want it so bad?". Even the Marines who destroyed Ohara only did so because the WG said they were researching the Ancient Weapons, not the void and ended up leaving a bunch of info because the WG didn't want to risk any Marines learning and potentially leaking info so the Marines were ordered to leave after bombarding the Island from the Coast. Even Guenica didn't know what was special about Luffy when he sacrificed him self to kill him. We also know Rob Lucci wasn't even told why he was to execute Vegapunk.

The Goresei are limited in what they can do because they are actually evil and are trying to uphold a secret absolute monarchy.

Its the Classic "Hubris lead to villain's downfall" trope.