It is simple, if you know what is really going on. Everybody who is on these lists, also known as safe lists, knows that they will be receiving emails from other members. This is possible because that is one of the conditions of their "safe list" membership.

And those who join these lists are willing to agree to this condition because they themselves would want to send out their own commercial emails to the others on the list.

The result: everyone is sending emails to each other but no one is reading them!

It gets worse when some savvy members sign up for the membership using a free or less-frequently-used account to store the useless emails they will never bother to open and read.

Having said that, it is always wiser to start your own mailing list and build it with opt-in subscribers, no matter how tempting safe lists can be or how many members there are in a safe list.