It's not that our ECU's are too smart, well it kind of is.
It is useless to reflash an ECU on a normally aspirated engine unless that engine has a large displacement and/or is ineffecient.
See the manufacturer already programs cars with small displacement engines to get the best balance between performance and mileage.
Now small displacement turbo cars is another story. You can see night and day differences when flashing the ECU on a car that comes with a turbo. For example: mk4 VW 1.8T can gain 45-55 whp by simply doing intake exhaust and ECU reflash.