There are definitely technical reasons why saving mid-run is a lot more complicated. With Pacific Drive, right now when you save, it'll save:

• the state of your car - this will likely be done by looking each individual "equipment slot" the car has, assigning them a number, assigning each possible upgrade for that "slot" a number/letter, and storing its damage state (which is probably just a scale of 1-5 or whatever). So the game will store everything about your car in the format off "slot x, upgrade type y, damage z", which can just be three values.
• your quest state. The game won't remember what quests you've done or how you've done them in the way that you remember it - it'll just store that you've completed quest step 14a and that 14b is your active objective.

It makes for a fairly simple, small save file. Being able to save mid-run would add a lot of complexity because it'd need to save a complete map state, including:

• the map layout
• your position in the map
• the enemies and hazards in the map - their positions, states, etc.
• what's happened already in the map
• the loot in the map, and whether you've collected it or not

And so on. Not only does it massively increase the complexity, it would also increase the size of save files a lot and make saving and loading a lot more cumbersome. And that's just a simplified breakdown; there are definitely other factors that can make it much, much more complicated.

There are definitely some games where "easy mode" save systems could be implemented without much changing on a technical level, but I don't think Pacific Drive is one of them.