Bale moved back to Tottenham in the summer after agreeing to join the club on loan from Real Madrid for the season.
The 31-year-old forward, however, has struggled for action since returning to Spurs and he has started just two Premier League games during the current campaign.
Asked why Bale was not playing regularly in his second spell at Spurs, Barnett, speaking at the Financial Times’ Business of Football event, said: “He’s towards the end of his career. Really, you have to ask Mourinho that.”
Barnett then moved to defend Bale’s record. The Welshman won two LaLiga titles and four Champions Leagues at Real Madrid.
“When they say: ’What’s happened to him?’ – he’s won more trophies abroad than any British player in history,” Barnett added.
“He’s done very well financially, and he has enough money for the rest of his life. He has a very good lifestyle, so that is what has happened to him.”
Bale came through the ranks at Southampton and joined Spurs as a 17-year-old in 2007 before sealing a big-money move to the Bernabeu six years later.
Addressing his initial transfer to Spurs, Barnett, who has overseen the Welshman’s career, said: “With Gareth we always knew where he wanted to be.
“We knew he had the talent so when clubs came in, the first thing we did was look at the clubs that were there and the ones to progress his career.
“We chose Tottenham for clear reasons; the path to the first team and good coaching.
“We turned down two offers for more money at the time but we spoke with him, his parents and they listened to us and he ended up at Real Madrid.”