Why are used cars you buy from a dealer so expensive ?

They charge what they think buyers are prepared to pay, without making themselves uncompetitive; but also, unlike the private seller, the cost has to cover their overheads before any profit.

That is true of any business - it has taxes, building costs (capital, lease or rent, utility bills, maintenance), wages, insurance, stock-purchase costs, business-vehicle costs, advertising fees..... It soon adds up.

I’ll pay extra to a dealership to avoid the scammers that sell privately. Too many where I live

Most dealers have recon centers. Essentially, they recondition used cars including paint touch ups, detailing and more. The cars look great and that drives up value.

Presumably you get some warranty or other service from the dealer. Often they also arrange finance and insurance.

Unless it is a reputable dealer, you are better off just buying from a private seller.
Avoid, like the plague, "buy here pay here" aka "second chance" lots. Ridiculous markups for junk cars, and of course no warranty.

Reason why they mark them up to at least double what they are worth is because they finance and they know most people do not have a lot of cash on hand to buy from private parties.

It is a rip.