With a combined income of 4000€ without children, saving 1500€ should be possible, right?!
How do you know so precisely that there was no inheritance/gift involved with your acquaintances? Grandparents, childless aunt...
But saving is a matter of personal attitude. I don't want to step on anyone's toes, but there are people for whom it doesn't matter how much they earn. They always manage to spend the money.
Example: our friends, same age, comparable income - difference: we had children much earlier (the first one 7 years earlier, the second one 5 years earlier). They may have about 10k€ emergency savings to this day, we have contributed 1/3 equity (>200sqm in BaWü under construction) ourselves - without inheritance and gifts. We also went on vacation, etc.
There are so many little things where you can save or not. A household budget book might be advisable for you to see where your money is going.