Game restarts instead of back to menu

[constantinople]

I mean, we've all accepted that the game hast to restart every time you want to go back to the main menu but- does anyone actually know, why its programmed that way? Its such a curiosity, Ive literally never seen ANY game does this. Like... there are literally buttons saying "back" that just dont work and restart the game instead. So strange. Has PD ever said anything about this?
(And please refrain from pure speculation and guesswork.)

[grotaclas]

A lot has been said about this. For example:
Johan in December 2015[forum.paradoxplaza.com]:

Its not a crash, its a complete reset of databases, instead of the former partial ones that always created bugs.

Or Mathieu Ropert(AKA French paradox) on Apr 9, 2019[forum.paradoxplaza.com]:

The "return to menu" is a legacy from EU3 in which going back to the menu after you start playing will result in a different checksum.
In EU4 it was quickly addressed by the "exit and restart to menu".
We have been wanting to fix it entirely for quite some time now but I can't make any promises as the cost is expected to be high and we have to prioritize it along many other things we could also look at (and, of course, the gameplay content of the next big patch).
Then again we also thought that we would never manage to make hotjoin work and in the end we did, so who knows?

IIRC there was also a dev diary from around that time which said that they are actually attempting to fix these problems and then a few months later they said that they were unable to fix them and the restart will remain, but I can't find them right now

[Marquoz]

Because it wrecks the game if it doesn't. Not restarting causes the same problems as if you looked at other starting dates before launching a new campaign--it breaks the game.

Fixing this problem was apparently too labor-intensive, so they implemented the current system instead.

Edit: lol, grotaclas and I posted almost simultaneously.

[Totally Innocent Chatbot]

It's a deliberate design decision. The game's engine has a tendency to cache things that can completely break various aspects of the game, and the only way to clear them is a full restart. You can see this for yourself by simply clicking on a late start date or saved game-in-progress to preview it, then clicking back to 1444 and starting a campaign - nations' starting government ranks, subject interactions, and all sorts of other random things will be completely broken compared to what they're supposed to be on a normal 1444 start. Now imagine how much worse things would be if you could exit to main menu from a live campaign and break things on a fresh save.

Rather than go through the effort of overhauling the ancient and decrepit Clausewitz engine to fix these issues, Paradox long ago applied the much simpler band-aid of making the game restart itself every time you return to the main menu instead, purging all the cached assets that cause these issues. Other contemporary Paradox titles, such as Crusader Kings 2 and especially Victoria 2, suffered from similar issues; newer installments like CK3, Vic3, and Stellaris no longer suffer from this problem for the most part, hence why they don't go through a full restart every time you return to the main menu.