[khuey]

For drugs that are new enough not to be available as generics, insurance companies typically have a "formulary" that specifies which drugs they will pay for and what the requirements are for them to pay for it. It might specify conditions like "we will only[0] cover drug Y from the class of drugs X" where drug Y is manufactured by AbbMerquibb and has negotiated a good price with the insurance company, or "we will only pay for drugs from class X after the cheaper frontline treatment Y has been tried and failed".

Doctors prescribe specific drugs. They don't prescribe "whatever SLGT-2 inhibitor your insurance plan covers". They also don't prescribe "whatever your approved first-line therapy for rheumatoid arthritis is". If doctors prescribe without caring about the cost to their patients, the best case scenario is that those patients come back for another prescription after the pharmacy tells them that their drug will cost $$$$ because insurance is not covering it. The worst case scenario is that the patients don't come back and decide they can't afford to have their condition treated.

[0] Barring patient intolerance to the preferred drug or various other exceptions.