Why? Well, Kraft offered a succinct four-word answer to reporters on Wednesday morning at the owners meeting in Atlanta.
"These are voluntary workouts," Kraft said.
Kraft noted Brady's family responsibilities for his absence. He also said that the three-time NFL Most Valuable Player is "very excited" about the mandatory minicamp, which is scheduled for June 5-7.
Gronkowski's absence from OTAs also wasn't a cause for concern for Kraft.
"I'm pretty sure he'll be there, as well," Kraft said.
Kraft's comments come one day after Patriots coach Bill Belichick was mum with reporters on the absences of Brady and Gronkowski.
"Yeah, I'm not going to talk about the people who aren't here. Guys that are here are improving, they're working hard and those are the guys we're going to focus on," Belichick said.
Multiple media outlets have reported that both Brady and Gronkowski are seeking new contracts ahead of the 2018 regular season. Brady has two years remaining on his current deal and is scheduled to earn a base salary of $14 million in 2018, while Gronkowski also has two years remaining on his current contact -- which pays a base salary of $8 million.
Brady committed to play the 2018 season during an appearance April 30 at the Milken Institute Global Conference in Santa Monica, Calif. He said at that conference that he desired to spend more time with his family this offseason while still preparing for the 2018 season.
Per the Boston Globe, Brady has been seen training both on the beach and on the field at UCLA. The newspaper also reported that he is expected to invite Patriots wide receiver Julian Edelman and Gronkowski to Montana for additional workouts in July.
Brady threw for a league-best 4,577 yards along with 32 touchdowns and only eight interceptions last season.
Gronkowski initially raised doubts about his playing future mere moments after Super Bowl LII, with rumors circulating of a career in acting, professional wrestling and even dirt-bike riding for the affable star.
He posted 69 catches for 1,084 yards with eight touchdowns over 14 games in 2017.
A second-round draft pick of the Patriots in 2010, Gronkowski has been a touchdown machine since joining the team, reaching double digits in five of his eight seasons, including a career-high 17 in 2011.
Gronkowski has surpassed 1,000 yards four times and has 474 receptions for 7,179 yards in 102 career games (89) starts.