Marvel and DC comics create new characters all the time.
The problem is nobody really cares unless they have name-brand recognition.
Usually, they show up in an event as part of an attempt to build interest, sometimes as a backup story on one of the mainstream heroes, and if they're lucky they might get a solo series that gets cancelled after 25 issues due to low readership.
Most of the time "new" heroes will piggyback on that brand recognition (Ms Marvel taking Carol's old code name, Miles Morales becoming Spider-man etc), but even then those don't always take off.
Jaime Reyes was the third Blue Beetle, but his first solo series didn't make it to 50 issues. He didn't get popular until later though apparently he had a die-hard cult fanbase.
I think the issue is that Superheroes rarely retire, or don't for long. Even then they are just replaced by someone else with the same codename.
Manga on the other hand have endings (unless you're One piece) so new series come out to fill the void.
Dragonball ends, in comes Naruto. My Hero Academia has been one of the recognisable brands of Shonen Jump for a decade, but that series will soon end and be replaced by whatever fills it's slot.
Even then I doubt you can recognise a lot of the series that didn't take off.
But for example, Batman is never going to end. And so there will never be a void to fill by an up-and-coming hero.
New characters might get a cult following, and maybe sometimes even break through usually off the popularity of a movie or TV appearance, but the battlefield of event comics are littered with the corpses of heroes that nobody cared about.
No seriously, sometimes they get killed off in an event as a sacrificial lamb so they don't have to kill off the heroes people know.
And yes, as someone who exclusively reads Spider-Man, I am part of the problem.