Your perception of Google software engineers is way off. They’re more often than not some of the best software engineers in the industry because their hiring bar is very high, and they get paid like it. YouTube is an astounding complex problem to solve with thousands of moving parts and non-trivial problems. It’s honestly astounding people are able to build sites that complex, and that they’re not only common but extremely reliable.
The issue is there are even more extremely intelligent software engineers outside of Google than in, and many of them spend some of their free time working on FOSS projects including ad-blockers. It’s also almost always harder to be red team (attacker, or the ad-blockers devs) as opposed to blue team (defensive, or the people trying to stop them).