You got some answers about visibility and safety, but let me add some details about the tires.

Wet tires are meant for conditions where it's absolutely pouring down. They can disperse 85 liters (~22 gallons) of water at 300 km/h (~186 m/h) per second per tyre. That's a crazy amount of water, and conditions aren't bad enough usually to warrant this.

And then if it is warranted, you can ask yourself whether it's still safe due to visibility and low grip due to cold tyres.

Intermediates can displace about half of the water at 300 km/h per tyre, and this covers it most of the times.

This is only guess and speculation

1. visibility. When its wet enough for the full wets, the drivers can't see well enough to be safe. They are not going to slow down just because they can't see.
2. general safety. When it is wet enough for full wets, the cars are so low they can still easily aquaplane unexpectedly.
3. the show. If its wet enough for full wets, they're going to be fighting more with the car than each other and the spray will make the cars hard to see.

Mostly it’s down to stubbornness. But under the guise of visibility. As Martin Brundle once said, they could always just go slower if they can’t see. But that’s not how racing drivers are built it seems. So they all go flat out in the wet, complain that they can’t see shit and then the officials red flag.