There is actually a reason for that. I don't have enough time to write a thorough explanation but to try and sum it up:
We instinctively live in te present. Goes bak to cave man days, living day To day to survive. No "I'll do my hunting tomorrow." You need to eat today do you hunt today. That instinct is still some what in us. If present you doesn't want I diet you say future me will start dieting tomorrow. But then when tomorrow comes, present you is there and present you still doesn't want to diet so you say future you will start dieting next week. Then next week comes and there is no future you it's still present you and present you still doesn't want to diet. So on and so on. If you want to do something you have to start today to really make it work because future you will always be in the future not the present.
Does that make sense?