I am going to make a good-natured quibble with your wording. I do not believe people stopped respecting the environment; I believe they never respected the environment.
I grew up in a very different world from what I see today, and I think things are much better now. When I was a child, my family would sometimes go to a favorite picnic area near a babbling brook, with plenty of nice picnic tables for everyone. Whenever we arrived, all those picnic tables would be piled with the débris from previous visitors. When we drove down the roads, there was litter everywhere. I think things are much better now than they used to be. Lots of people have changed their attitudes dramatically over those decades. The bad attitudes that shock us today were the norm when I was young.
But despite that change in general, lots of people have not changed their attitudes. I believe a lot of people are still raised in homes where trash is left on picnic tables or tossed out of car windows. Those people never had respect for the environment, so we cannot expect them to get it now.
About 45 years ago I was a very naive young adult on my first visit to the beauty of Rocky Mountain National Park, which we were visiting with my wife's mother. I was only a few years out of college, where we had celebrated the first Earth Day, and where I had this developed an absurd belief that I was part of a new generation "with a new explanation". I watched a small boy crouching down, trying to attract a chipmunk with bread crumbs while his father, a man not much older than I, watched with a broad smile on his face. What a beautiful scene! When the chipmunk got good and close, the young man whipped his arm out from behind his back and hurled a rock at him. "You almost got him that time!" his father said approvingly.
A little later we were at a lookout in an alpine meadow above timberline. There were signs all over the place warning people not to walk on the delicate tundra, and people were dutifully crowded onto the path. A young couple, again about my age, trudged right through the delicate wildflowers. My mother-in-law, never one to hold back, politely reminded them to stay on the path. The man looked at his companion and said, "For $10 I'd **** on her." They laughed and went on.
So, yes, things are much better now than they were, but there are still those whose motivations and attitudes are alien to the rest of us.