> Think about it - if these formations were not leakproof on geological scale, there wouldn't be any oil and gas for us to extract in the first place.

This is survivor bias. We only see the sites where it didn't leak too much, and extract from it. The sites that leaked, we don't hear about them (no oil to extract). In fact, a quick search shows that there are many leaking sites (both on land and under the oceans). The argument you present is a case of survivor bias, especially on the time scale of multiple millions years.