For years, the Giants have had a problem in their outfield.
Last year, the Giants outfield was ranked 28th in the majors, with a total fWAR of 0.1 (not a typo!). In 2017, they were last in the majors with an fWAR of 0.6. In 2016, a year in which they made the playoffs, they were 26th, with an fWAR of 4.3. They were 24th in 2015, and worth 5.3 wins.
You have to go all the way back to 2014, a year in which the team’s outfield fWAR was 8.9 (good for 12th in the majors), for the Giants to have had an above average major league outfield.