Since we lack the experience, memory, and knowledge of how things used to be, we cannot perceive how much our environment has changed since we compare it to our own 'normal' baseline and not to historical baselines.
This is a problem in environmental conservation, but also generally in terms of how we see our world. We don't know any better than our current situation.
References:
- Are You Suffering From Shifting Baseline Syndrome? from Reagon Pearce on Earth.org