Why is maintaining a lower weight harder?

New here, I’ve read the rules and I hope I’m not breaking them by asking this. 33M, 5”7, 74,7kg when I last checked, about a week or so ago (Went abroad for Christmas for a month and put on about 4kg, have since dropped 2). My goal is to get down to a range of 64-67kg I was talking to my brother about it and he told me to just do a water fast or something for a few weeks straight, I laughed it off because everyone knows if you do a fad diet you will just put on the weight again. But then he pointed out that I don’t really gain weight and I’ve been in the same weight range(72-75kg)for about 10yrs. I eat cleanly enough, prep all my meals (proteins, lots of veggies and fibres, minimal/no carbs but I do like snacking a lot 😩), at the gym 4 days a week, decent amount of muscle and all that. So my lifestyle and my body’s metabolism seem to be at an equilibrium. His point was if I’m able to maintain this weight range, why couldn’t I do it at 10 kilos less? Just suffer through a few months of hard cutting, then go back to normal. Based on everything I’ve ever heard or researched about weight loss and healthier living, I know what he is saying is BS and not at all the way to do it. But I can’t actually explain why it wouldn’t work?