Because they are bought in bulk (by the pallet) and delivered at freight rates. It’s common for folks who need a lot of them to buy a pallet, take what they need, then sell the rest.
because they are leftovers after a job
the really cheap ones (like mine) are commercial or residential pulls Cf. WIll Prowse’ $50 250w pulls
Also why is it that panels are so cheap off the pallet? I’m seeing a lot on the sites you mentioned whereas they’re much pricier ordering online.