Why do many GI specialists insist on using fentanyl/versed as their apparent universal sedation methodology, including arbitrary follow-on dose?

Conscious sedation. Versed and Fentanyl are a standard combination of medications use for light-moderate conscious sedation when administered by a non-anesthesiologist. Most facilities require that an anesthesiologist (trained in intubation) be present for administration of Propofol (the drug used by Michael Jackson with clearly bad outcome.)
It's effective. It is effective for sedation and pain control and is safe without too many adverse effects. Both can be reversed easily if needed. Also, it's what everyone is used to using and that is beneficial to the doctor, the nurses and ultimately the patient. .
It's the easy way . Most GI specialists have found that a standard dose is usually safe and effective when using those two drugs. However not all patients are equal. Some may require no additional meds but others require much more. That would require an anesthesiologist to administer propofol either as a bolus or via a pump and additional doses of fentanyl and midazolam. Safety and patient comfort are paramount. .