There are two incorrect assumptions going on here:
1) Costs will stay the same.
These are novel therapeutics that recently hit the market and where there are tremendous imbalance of supply and demand. Costs will inevitable come down when competition hits the market (as Ranger said) and as the supply chain gets ramped up (Novo just bought Catalent to literally convert all of its production for this one product).
2) Drug makers want these high prices.
If you are Novo, you want the price to get lower so that you can get more and more people on the drug. These are not short-term drugs. Everyone who goes on them is ultimately reliant on maintaining their usage or there is an extremely high probability of weight reversion. They'd love nothing more than to quadruple their customer base that is hooked for the long-term, even if they can only charge $500/per vs. $1,000.