Ha. It is hilarious to read students/residents amazement at how one could spend so much money so quickly. It happens. Every year I look at my tax return and am amazed at how much I made, and what it felt like we had in reality.
First taxes become significant, for people that dont actively work to figure them out or live in high income tax states you will lose around a 1/3rd of your income immediately. You arent "making" your salary in the same way you do in residency. Next, if you have student loans this can be significant and is 100% post tax so tack that extra 1/3rd amount you have to make to paying them. You are out a decent amount already.
Then comes the stuff you have more control over, home, cars, food, etc....many people hurt themselves here yes, but its usually due to not appreciating the impact of the first 2 issues and thinking they actually have a whole lot more money than they do. This leads to underfunding their retirement and voila, under accumulators of wealth.
Even for people that consider themselves frugal and focused your regular bills creep up a tiny amount at a time and together become more, eg, you no longer can stomach mad dog 20/20 and buy slightly nicer wine, that adds up. Healthier food choices, more expensive. Im constantly asking my wife how on earth we spend so much on groceries, chill on the wine, but we other wise eat very healthy and a lot of produce which adds up and isnt worth cutting out. Even if you're not going crazy the quality of little things likely creeps up over time.
The good news is if you're smart and dont put yourself into too large a hole early on where youre beholden to debt or a job you dont like. Things can then turn around rapidly after living in an intentional manner for the first couple years. For example, instead of going for the amount of house our lender/realtor approved and wanted us to buy we will be closing in a few days on a house that is about 2/3rds my annual income and will be transitioned into a rental in 2 years (5 if I can get the wife to love the low low mortgage).