Because they’re no more than average players at best, combine that with the huge pressure they placed on themselves, simply by being Brazil & hosting the tournament.
This was the weakest team in memory, certainly since the terrible sides of the 74/78 World Cups, they were always going to be found out & were ripe for a hefty beating.
Missing Thiago & Neymar is just papering over the huge cracks in a side that were running on passion & hope as much as anything, that gets you only so far at the top level in any sport.
The fallout from that defeat will be massive, Scolari will be gone, some of the players will never pull a Brazil jersey on again. It mirrors what happened last time Brazil hosted the World Cup in 1950, they lost to great rivals Uruguay & years later one of the players wrote a book, in it he spoke of the players shame & that most of them went into hiding & never recovered in their careers.
They were absolutely slaughtered by the media & fans, forever tarnished by their failure, that’s how passionate Brazillians are about their football & losing that badly in a World Cup they hosted, the scars will run deep.