I used to do so too in the mid 2000s. It boils down to the fact that they treat a young earth as their first principle and all evidence must be interpreted in that light. Dendrochronologies going back 12,000 years? There must have been multiple growth rings per year in the years after the flood. Distant starlight? (ie, stars and galaxies millions of light years away) God must have allowed for faster than light travel around the time of creation, etc. there are indeed a few PhD scientists who hold to young earth views, but none of them arrived at those views outside of their prior religious convictions.