It’s about profit not the environment. They just say that so people will go along with it. Some people are willing to pay more if they think it “helps.”
In reality now someone is going to have to take one off of the shelf and buy it. That one is wrapped in packaging and had to be shipped. Not everyone wants or knows about or cares about aftermarket chargers.
Apple ships 200 million iPhones each year. That's 2 billions for the next decade. iPhone users will not need to buy 2 billion chargers the next 10 years.
If you buy a charger today, you can still use it for your next 2-4 iPhones and now people are not expected to get a charger when you sell it or hand it down, since Apple no longer includes a charger.