
The story occurs in a year and a couple months, this is incredibly fast paced but for the story it's trying to tell it absolutely works in its favor.
Imagine if in real life most people had powers and being a normal person was the minority, there would constantly be chaos and unrest that doesn't ever really go away even when things appear to be under control. Nothing would feel stable. The series reflects this, it's fast paced, chaotic and not truly stable despite appearing to be on the surface.
The villains waiting around for three years would also heavily undersell them. I can't ever see Dabi waiting three years to get his revenge when Shoto starts embracing his fire side and starts becoming what he never could be, a hero that makes his family proud. The same goes for AFO, him waiting for Deku to get stronger and stronger would just feel like demon slayer levels of plot armor and he'd be getting dunked on for being a bad main villain.
Twice would have a lot longer to recover mentally and become a near unbeatable villain, him taking three years to do this and then instantly getting killed by Hawks would feel lame, because it would be.
The villain's would straight up have to be rewritten a lot in order for the story taking place over three years to even make somewhat sense. Deku's progression would need to be rewritten and it's pretty much a different series from then on.