Because the envelopes come addressed to both of us.
Her name is printed above mine on every single one of them. In capitals. In the window. Where you cannot possibly avoid it in the four seconds between the mat and the table.
It is a joint account. That is simply what a joint account does and will go on doing. There is no malice anywhere in it and it has been doing that to me on the fourth of every month for ten years.
There are four boxes of them.
Unopened. Under my own bed, about eleven feet from the kitchen table with the tin of soup on it.
I put them under the bed because the drawer in the hall filled up. In about 2019.
That is the entire thinking behind it.