I always assumed it's because physics is a little against synchro teams when it comes to death spirals. Just think of the size difference in most pair teams
I think it's because it's easier to do with little or none size difference. But there are also teams that do it more properly, like Hockettes junior : https://youtu.be/FMxTd6dXwj4. It' the first element in their program. There might be also a problem with teaching the proper technique that the synchro coaches just doesn't have the knowledge how to teach the death spiral. And also the mentioned way to do that is ok by the rules.