There's a non-motorized version in PBR too, but is set to be used only if the airplane door's height is less than 2.70 mt from ground. You are using a 737, which has a door of about 2.90 meters from ground, so you don't see it.
We chose this threshold because the non-motorized version doesn't have any human character, which is something we surely would like to add but, in the meantime, we wanted to show off our nice new PBR people to users of the popular 737s. We might change the threshold height in a future update, when we would need to show off a new human pulling the stair manually, which is surely planned.
You can alter this by changing the relevant line in this file:
Addon Manager\Simobjects\PBR\FSDT_Staircase_CDS_2438\sim.cfg
to read like this:
condition = ( %(exitAlt)f >= 1.80 ) and ( %(exitAlt)f <= 3.2 ) and ( %(isCargo)f == False ) and ( %(isCargoStand)f == False )