I suspect three reasons:
1. Cooktops in islands need ventilation and downdraft systems are notoriously ineffective (I've heard lots of complaints here about them, as well as a few people rising to their defense).
2. If you don't use a downdraft then you need a ventilation hood floating in the middle of the room. Not everyone likes the look of this, and it is harder to vent to the outside this way.
3. Things splatter when you cook and if people are sitting across from you at the island, that's potentially unsafe and messy.

In addition to the main reasons/ feeny's comments, the downdraft systems that do work pretty well are quite loud, as are the ceiling mounted so you will have to talk over the noise. Also, it is less expensive to put it along the wall, so builders are probably showing it as it would be at that price point, vs. upgrading. If you do decide you want it in the island, shop for appliances somewhere that has working hoods so you can hear them. Also make sure your island is large enough to put several feet between the cooking surface and seated guests.

I am having a new house built and the builder has cooktop facing wall. Has sink and dishwasher in island. And it is a huge Island. I am happy with this choice as I have small grandchildren and with the gas cooktop I would be worried about them sitting on the island seating area. Originally I to thought I wanted it on the island area.