Just to state the facts - Biggio has gone from the 100th percentile for chase rate (in 2019 and 2020) to the 81st percentile so far in 2021. Is he trying to be more aggressive and drive in runs? or is he simply pressing a bit after a slow start? I'm not sure anyone on this board knows the answer to that and it's not like it's fallen off the cliff. His incredible eye allows him to target pitches he can handle/drive and that's the root of his success.
Gurriel on the other hand may have all kinds of physical tools, but he has no eye or plate discipline at all. He's in the 4th percentile for chase rate right now and his career best was the 17th percentile last year (12th in 2019). There's typically no reason at all for a pitcher to throw him a strike - or at least anything that isn't on the edge of the strike zone. Unfortunately, that leads to him having to hit 'pitchers pitches' instead of pitches he can handle - while doing damage anytime a pitcher makes a mistake by throwing him something near the heart of the plate. This makes him very streaky. When he's going well, the mistakes end up getting hit into the gap or over the fence. When he's cold, he misses those mistakes.
Biggio and Gurriel are polar opposites, but I think you discount the 'tool' of plate discipline and the advantages it provides hitters.