The answer to your first question about preconceptions is that scientists are human beings and so scientific practice is affected in many ways by human subjectivity. These effects include, but are not limited to, a human tendency towards dogmatisim. Scientific methods and training can limit the damaging effects of human dogmatism, but cannot eliminate them entirely.

The answer is your second quetsion about intelligent design is that there are substantive scientific reasons for rejecting this argument for God's existence. Ironically, your assumption that scientists' failure to accept this arguments can only be due to unscientific dogmatism may be based on your own wishful or dogmatic thinking -- it is a mistake to think that the best or even the typical scientific response to the argument from design is dismissing it as a childish myth.

Finally, my sense is that scientists and philosopers have explored and assessed this argument from mutliple perspectives, so I think the rational investigation you advocate at the end of your quesiton is underway.