Because wholesale means trade-only. Manheim's whole deal is dealers selling to dealers at trade prices; open the doors and it's just another retail lot. To get in you need a state dealer license — and that means a bonded application, an actual commercial premises, insurance, and renewals. It's a small business, not a weekend permit.
Realistic options for the rest of us: buy from independents who shop those lanes (and check their prices against wholesale comps), use a licensed broker or a managed bidding service with published fees, or bid the public salvage lanes at Copart and IAA where your state allows it.