It's the racket of big pharma. Ketamine isn't approved for the treatment of depression but Johnson & Johnson's patented Esketamine is. If you want to go the legal route that is your only option.
Depending on how much your insurance plan covers you, as a patient, might only pay a fixed copay with your insurance covering the rest, so you might not actually "feel" the $7k price tag at all.
The real scam that has played out over the last few decades though is that as employer insurance costs rise wage growth has slowed. We've all been paying for rising insurance costs even if we don't necessarily feel it.
It's really a form of regulatory capture and goes way beyond drugs into medical devices, IT systems, etc. The "approved" thing is often a more expensive, less complete/reliable/modern version of something people who aren't subject to regulation can get off the shelf. You can think of it sort of like if car dealerships managed to get a law passed that said you can only service your car at the dealership, and then 10x'd their prices.
Because the penalties for unauthorized possession in the US range from (up to) 6 months to 8 years with fines up to $25,000 varying by state and the only way to acquire it in an authorized manor restricts it's prescribed usage to a narrow scope of conditions, the treatment of depression not included.