If it were really easy then why do so many people fail at it?
It's easy in principle but many people don't understand what compound interest does to them. Credit cards make it so easy to spend $1000 and pay back $35, and then 28.6% interest takes the remaining $965 and turns it into a steady income stream for the credit card company.
It's like the simple process of losing weight. Eat less and move more. It's about as simple as it comes but when it comes to actually taking in fewer calories it's not always as simple as it seems. You might use the ultra lean meat and fresh vegetables to make a taco blend, not realising that the taco itself contains 200 calories or more. And of course the way food manufacturers helpfully provide detailed dietary information based on a "serving" when what they call a "serving" is a small fraction of what you'd expect someone to actually serve, gives people the impression that they aren't taking on as many calories as they are. It's easy to see, for example, the calorific content of the carefully defined 30g of breakfast cereal with 1/2 cup of skimmed milk and not see past that to realise that a bowl of cereal is actually 105g (i.e. 3.5 servings) and a cup of full-fat milk doesn't bear meaningful comparison to 1/2 cup of the watery rubbish known as skimmed milk. While the carefully defined portion might only contain 150 calories by the time you've poured out three times the notional serving size and used milk rather than watery milk you could easily end up with 600 calories, all the while thinking you'd only taken on 150.
To top it off there are so many exercise apps these days that give horrendously unrealistic estimates of calories burned. I'm a big heavy guy and the most accurate estimates I've been able to calculate (by measurement, power meter etc) is that I'll burn up to 200 calories per mile walking and maybe 30-40 calories per mile cycling (averaged out, obviously freewheeling downhill uses very few and hauling myself up a hill uses more). Yet I've seen people half my size quote figures up to 120 calories per mile while cycling. If you cycle 10 miles and think you've burned 1200 calories when you've actually burned maybe 200 you've immediately got a 1000 calorie mismatch between what you think you've got and what you've actually got.
