Because science is incapable of proving something. It only tests theories for falsification. So far, evolution has not been falsified every time it has been tested. Most of the science stuff from the DI is so bad, one comes away less informed after reading it.
“So far, evolution has not been falsified every time it has been tested”
The evolution model should be verifiable through the fossil record, yet there is no proof, represented by transitional life forms. Trying to protect your inability to prove that by a falsifiable model standard speaks to how valid your “science” is.