Because you can't think things through enough to make sure you're quoting one line.
4 lines on Premium (what you quoted above) is $40.99/each with autopay and signature discount on AT&T. Still $15/cheaper per line.
4 lines on Value Plus PL, $30.99/line for 4 lines. But hey, you actually get more than $450 trade in credit, unlike with Verizon.
It doesn't matter if YOU don't trade your phone in. The vast majority of the population does.
I do this stuff for a living and you think you know more than not only me... But also my colleague who works for Verizon.
I laugh at you.