I don't understand retentions logic

I'm booked on for disconnection in just over 30 days.

I've not had any calls - I see from other threads that it's a bit hit and miss.

I'm going to go openreach which has about a 2 week lead time for install.

From a business perspective, why would virgin leave it so late to try retain me as a customer?

Once I've got my install date I'm not going to accept any offer from virgin.

I can only guess there's more people who don't negotiate at renewal , so overall it pays off for them to be a shitty company?