The 34-year-old striker, back in the England squad for the first time since November 2013, said ahead of Wednesday's friendly against Germany in Dortmund that he had to be realistic.
"To get into any squad, it's based on merit and you've got to be playing at the top level. It's as simple as that. When I went to Toronto, I missed out on a World Cup," Defoe told reporters.
"The boys I'm competing against are playing in the Premier League and playing in Europe and probably that's why I missed out on the last World Cup. You probably need to be playing in the Premier League."
Sunderland are bottom of the table, seven points from safety, and even Defoe's enduring goal-scoring magic looks unlikely to save them this time.
The striker's 14 league goals make him the second highest English scorer in the top flight this season after Tottenham Hotspur's Harry Kane.
He was not included in Roy Hodgson's England squad for the 2014 World Cup in Brazil after joining Major League Soccer side Toronto in February of that year.
Despite joining Sunderland in January 2015 and scoring 15 goals to keep them up, he was also overlooked by Hodgson for Euro 2016.
Defoe, who has 55 caps and played for England in the 2010 World Cup finals, said he had never entirely given up hope of a recall despite occasional doubts.
"It didn't really stop me from getting my head down and working hard and just hoping one day I'll get an opportunity to represent my country again," he added.
"I was getting a lot of stick from the lads at the training ground because I've still got the England flag on my boots. They'd say things like 'JD, why have you got the England flag on your boots still?'".
With more than a year to go before the World Cup in Russia, Defoe believes he still has a chance.
"You just want it more and more. For me, I think if I do get an opportunity to go to another major tournament, it will be a dream."
(Reporting by Alan Baldwin; Editing by Robin Pomeroy)
