The Bystander Effect/Diffusion of Responsibility is a recurring motif in My Hero Academia. In the most recent arc we learned that Tenko Shimura grew up with an abusive father who would lock him outside for the night and beat him just for mentioning heroes. Tenko's mother and grandparents knew what was happening, they watched it, but did nothing to protect Tenko. Later on when Tenko Shimura is wandering the streets everyone ignores him, thinking it's not their responsibility and that a hero will come along and handle this kid.
This motif was introduced in the very first chapter of the series.Early in the 1st chapter we see Bakugo explode Izuku's desk in plain view of the teacher and the rest of the students. Izuku scampers backwards, terrified, as Bakugo stands over him yelling; at no point does anybody try to help. Contrast this moment with what happens in the Quirk Apprehension Test Arc when Bakugo rushes at Izuku screaming. Later in the 1st chapter we get a more explicit example of the bystander effect when Bakugo is taken hostage by the Sludge Villain. A crowd of people gathers, not to help Bakugo but simply to Gawk.
During the Entrance Exam arc after the Zero Point Robot shows up, Izuku is sitting on the ground frozen in fear of the approaching giant robot while everyone else flees. Tenya Iida clearly notices Izuku but does nothing to help him and keeps on running, he presumably also noticed Ochako Uraraka (whose legs were caught under rubble) but did nothing to help her either.
From Tomura Shigaraki's perspective Hero Society has lulled people into complacency and a false sense of security, now that people are "Holding out for a Hero" they would rather let heroes deal with suffering people than actively help them themselves. Thanks to the existence of All Might even the Pro Heroes were able to take it easy.
This is a legitimate critique of Hero Society but it's important to remember that Shigaraki isn't really interested in building a better world.