Well, there are a couple reasons.
1. Need of a loaner car. When factoring in the cost of the loaner, the service doesn't cost that much.
2. Desire to have a complete maintenance record on file at Lexus. Lexus has a national database that has your entire maintenance record (when serviced at Lexus), thats worth a lot at tradein time.
3. Desire to have only genuine parts used. When you have your oil changed at the quick lube they don't use a genuine Toyota filter (found in independent tests to be one of if not the best). This can of course be remedied by bringing your own.
4. Desire to forge a relationship with a service advisor. I've found this is very important. Down the road when warranty work has to be done etc.
5. Or they just plain enjoy having their vehicle serviced at Lexus.
This is why I mainly have mine serviced at Lexus, mostly for the loaner car. They charge me $130 for an oil change and tire rotation and I get a loaner for the day, when figuring in the cost of renting something similar to a Lexus they're really only charging me $40 or so.