I can think of three reasons that fuel economy is worse now:
1. A/C is now so common that it’s difficult to find a car without it.
2. The cars have more powerful alternators because there are more electric accessories in them, and in the case of Canadian cars they had to beef up the charging systems to power the mandatory daytime running lights. Most cars need a couple of hundred watts more power than they did even a decade ago.
3. 10% ethanol blend is fairly common now, which reduces mpg on all cars (including older ones) by 3%.
4. Every car made today is heavier than the equivalent model from 15-20 years ago. Bigger engines, bigger wheels, side impact protection, airbags, etc, etc. All that stuff adds weight and/or increases rolling resistance.
You just couldn’t sell a 2000 lb car with a 70 HP engine, these days. I remember when cars like Honda Civics first hit 100 HP, and it was a big deal. Other than the Smart car and a few others on the fringes, I doubt if you can buy a car making less than 100 HP these days. Or without AC, as you say.
Unfortunately, the people who make decisions are generally scientifically/technologically illiterate, and really don’t know that you can’t make cars bigger and heavier with more safety features, and expect them to get more fuel-efficient at the same time. And then there’s the EPA and CARB - and again unfortunately, optimum cleanliness in an engine doesn’t coincide with maximum efficiency.