Marvel fans are probably reeling from where The Defenders left its characters by the end of the series. The Phase 1 wrap-up of Marvel's Netflix Universe has set up Daredevil Season 3, The Daughters of the Dragon, Iron Fist Season 2 and much more - but at what cost? They're the first group who will probably never see each other again. The showrunner has even said it's Unlikely they'll come together again. Why? They technically stopped the Hand and saved New York City, leaving all of them a little wiser and a little more aware of their place in the world around them. While the Avengers handle cosmic threats, the defenders seem like a perfect team to handle the friendly (or not so friendly) neighborhoods of New York City.
The fact is that no matter how far they got their division proved that they're better off leaderless, indeed that they're better off as a one-shot group of individuals who will probably influence each other but never fully come together again unless the city really needs them. Even then though? It's unlikely. Matt's recovering in a convent, Jessica found a degree of self-confidence and healing, Luke Cage returned to Harlem and Danny Rand took over Matt Murdock's rooftop patrols. Each one of them found a place in the city that they saved ready for their next adventure.
The fact is that if Matt had listened to Stick in episode 5, Take Shelter, and they had a leader, they might not have gotten as far as they had and the series would be far less entertaining. Why does it work for The Defenders as opposed to the Avengers or Marvel's Agents of S.H.I.E.L.D? Let's take a look at just why they function better on their own as opposed to under direct leadership.
THEY EACH BRING SOMETHING DIFFERENT TO THE TABLE
Leadership is a rare quality and those who become leaders don't do so easily. In the Marvel Universe, both Captain America and Phil Coulson have led teams with both positive and negative consequences. Captain America's leadership eventually clashed with Tony Stark giving us Captain America: Civil War and Phil Coulson's leadership in Marvel's Agents of S.H.I.E.L.D has been tested strenuously both by outside forces and Grant Ward, a member of his team who lied to Phil and tried to sow discord from the inside out as a member of Hydra.
It's possible that both Captain America and Phil Coulson could work well with the defenders, but we've had 2 years of getting to know these New York characters intimately. They're all extremely powerful but they're also very aware of their strengths and weaknesses. Matt Murdock knows that he's compromised by Elektra because of his faith and because of the law, Jessica Jones is painfully aware of her faults, and Luke Cage spent time in jail. The only person who initially tries to take charge is Danny, but even Danny recognizes that he's far too burdened to try and take charge of the so-called "team" even when Colleen suggests it.
They all know what they can do, but in knowing who they are they also know that none of them are fit to lead the group as a whole. That's different from both Steve Rogers and Phil Coulson because while they know they have faults they also see a clear threat. Dealing with lies and shadows and misinformation from the Hand, no one has a clear picture until it's too late. Matt fundamentally does not want to take charge, he's trying to break away from the vigilante life he's led. Luke knows where his allegiances lie and he's motivated to protect Harlem, apart from Jessica he has no real connection to the rest of the characters (as of yet.)
By the time Elektra and the Hand are in position, the team is thrown together without a common cause or purpose. While Captain America is here to save the free world and Phil Coulson is here to save the world from alien threats, the Defenders don't know what they're fighting until it's too late - and they don't trust themselves to lead the fight against it. Each one of them has faced their own demons and they're not prepared to take on any extra ones including the ones that come with saving the world. The fact that they do is a testament to four good people working together against incredible odds but with a leader? Things would have been far different.