True but you have to ask two questions:

1. when was it created. Things on the east coast are older and the administration of a county the size of Delaware would have been difficult in 1770s. They also took on a lot of historical boundaries that existed during colonial times.

2. A state like Nevada with big counties might make sense as it was a state that developed much later, and is much less dense.

3. even with the changes in advancement of transportation - who's going to vote to get rid of their own autonomy and be merged into a larger entity with neighboring counties that could have differ goals, budgets, etc.