This is ridiculous! Some of the levels take ages to complete. No saving during levels means having to start over from square one if you have to leave the game for some reason, or it crashes.

Or am I missing something?

Well, it certainly was a very moronic idea of the devteam way back then, to enforce such a player unfriendly system.

Hey, these are OLD games, take them or leave them. "Player unfriendly system" is putting it very mildly. Games used to be incredibly tough, many of them completely illogical and/or flawed or rigged to be downright impossible to win (cough, Mortal Kombat 2, cough). Maddeningly so. That was the price of admission back then. Being good at some game or even beating it was an actual achievement you could brag about.

That being said, the save system in Dark Forces was most likely intentional as by 1995 mid level saving was already a pretty common feature. It adds to the "realism" of being a New Republic mercenary. You go on a mission and you are on your own, no crutches, until you complete your mission objectives only then can Jan pick you up. I guess the devs thought having multiple lives was enough and also when you die you respawn close by and do not have to redo the level from start.

If you really need mid level saves play this on some console port on an emulator which has save state feature, I believe there is a PSX version floating around.

While it is true that other games from that time had a save/load system and only one player life in return,
Dark Forces had a different approach. It would not let you save, but it would grant you several lifes, so you could die a couple of times.
It's two different approaches for the same problem. Instead of Try and Error, DF uses a arcade style system.
Some say, this is the case because it was designed to run on Playstation as well, but that's just speculation.

And yes, compared to it's direct competitors Doom and Doom 2, Dark Forces' levels were huge. So yes, bring enough time, before you staret playing :)

At least DF saves the profile game between levels, I know a lot of games from that time (most platformers in fact) which would not save at all.