Why is it better to have less of a deficit the closer you get to goal weight?

Because you have less calories to take away from, since the less you weigh the fewer your maintenance calories will be.

You can't safely maintain a large deficit when you get close to your goal. It's not possible without risking your health.

When you weigh less, your body needs fewer calories: fewer calories to carry out its basic metabolic functions, fewer calories to move a smaller amount of mass over the same distance, etc. I used to burn about 100 calories per mile I ran; now, after reaching my goal weight, I only burn about 80 calories per mile.

So, let's say you used to burn 1700 calories doing your daily activity, and then you lost weight and now you only burn 1500 doing the same activities. If you eat 1400 calories per day and burn 1700, you are creating a deficit of 300 calories per day. However, if you eat 1400 calories and burn 1500, you create a deficit of 100 calories per day. Your weight loss will slow down even if your intake stays the same.

You *can* simply eat fewer calories, but only to a certain extent. 1200 is the recommended minimum intake for women, 1500 for men, and neither of those minimums include any exercise. The closer you already are to those minimums, the less room you have to create a larger deficit. And for many people, those minimums are simply not enough calories to fuel their bodies.

You also shouldn't be using exercise to create a deficit that puts you below the minimum calorie intake. It's not healthy or safe to, say, eat 1200 calories, burn 500, and then not eat those 500 back. That would mean you've only given your body net 700 calories that day, which is too few for any adult, let alone an active one.

So in summary, when you're close to your goal weight, you would be risking your health by trying to create a deficit that's too large. Better to stick with a smaller deficit and lose weight safely.

The short version is this: the more body fat you have, the more your body will feel comfortable safely tapping into your fat stores for additional energy. As you have less body fat, and you operate at too big a deficit, your body will think that you are starving it because it is going to deplete its fat stores too rapidly and will instead burn lean muscle mass to provide that fuel.