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Carb loading is not an excuse to eat whatever you want in the days leading up to an event. Carb loading is a strategic method of consuming specific foods to help load muscle and liver glycogen stores.
Over the past 50 years, a significant amount of research has shown the important role of glycogen for delaying fatigue in athletes competing in endurance and ultra endurance events.
Glycogen is the storage form of carbohydrates. Glycogen is primarily stored primarily in the cells of the liver (~100g) and skeletal muscle (~500g). Skeletal muscles transform chemical energy to mechanical work. Glycogen is the body's predominant source of energy during moderate to high intensity activity. The depletion of muscle glycogen during endurance activity causes early fatigue.
Carb loading should not be confused with calorie loading. Effective glycogen loading should be the result of emphasizing more energy-dense, carbohydrate sources and reducing fiber, protein and fat.
As you can see from the above examples, both have a similar amount of carbohydrates (which is around 8g/kg/bw for a 125lb athlete) but the carb load example is far less in calories. Considering that many athletes feel "too full, heavy and lethargic" when trying to "carb load" it takes strategic planning to make sure that you aren't consuming too many calories from protein and fat.
And by strategic planning, you need to consider where, when and what you will eat. If you are traveling, feeling rushed, lacking an appetite or relying on fast/restaurant food, you are likely going to struggle to meet your personalized carbohydrate needs before the race.
You need to have a plan for what, when and how much you will eat in the 24-48 hours before a race. This should be practiced in your training so you can determine which carbohydrate-rich foods work the best for you and how much/little protein and fat you need to feel satiated with stable blood sugar. If you don't take the time to figure out your pre-race eating in advance, there's a good chance you will end up undereating carbohydrates or overeating fat and protein, thus increasing the risk of underperforming.
A carbohydrate intake of 8-10g/kg/bw per day in the 1-2 days before your endurance event is recommended.
To read more about the topic of carb-loading, here's a detailed blog post.