Forget the ERA stats, Cubs bullpen WAR for 2019 was 1.3 which put them at 20th in all of baseball. They didn't get outs or hold leads when it mattered and you could probably pick apart some of the metrics from past seasons too.
https://www.fangraphs.com/leaders.a...nd=0&team=0,ts&rost=0&age=0&filter=&players=0
It's the same story with the offense, they were not clutch when it matters the most. They finished with a +97 run differential and it was somewhere over 100 for a good chunk of the season until they stunk it up late in the season. Look at their team stats in the NL:
OBP: 4th at .331
Average: 7th at .252
HR: 2nd at 256
Runs scored: 5th at 814
See what I am getting at, you can't just pick and choose stats and go "see on paper they weren't that bad!" You have to look at the overall work of the team and on paper this team looks good statistically in some categories but in reality both the offense and pitching didn't get the job done when it matters the most in the game otherwise they'd be preparing to play October baseball right now.