Though, in games like CK2 and EU4 , it is often essential, since some country might not even exist but at some specific dates.
For hoi4, it could bring some new challenges to start at specific dates. I wonder how come it is doable for game like CK2 / EU4, over a very long time span, and not for hoi4.
Because the setting for HOI4 is not over a very long time span? HOI4 is about a very specific small span of time, the start and end of WW2. Jan 1936 was probably the best date to allow the Chinese and Japanese players to set their nations for the 1937 Sino_japanese War and to include the SCW.
It's not about being doable or not, it's about allocation of resources. They could fix bugs and add new mechanics and features for everyone, or create and maintain a starting date for an infinitesimal part of the player base (and yes, this is a fact backed by numbers).
Can't say it for CK2, but the EU4 devs themselves have said that those start dates aren't worth maintaining as well, when the vast, crushing majority of players start at 1444 anyway.
I appreciate there is a very vocal minority asking for more dates. The issue is it is a very small minority. It isn't worth PDS spending time on it, as
@Vohen noted above and as the head of the HOI4 project team has confirmed.
1939 (or later) Starts is pretty much giving the player whatever was done historically by each nation and forcing them to fight with that. Most players think they could do better than history and would prefer to not make the same mistakes over again.
Earlier dates than 1936 bring in balance issues. The further you go back the more variables there are to take into account. Balancing for 1936 and the alternative paths PDS is adding o each major and it's satellites has got to be a royal pain. Even with all the testing PDS does before release people are still finding conflicts in focus trees from TvV and DoD. Adding another 3 years of focuses and ways to go ahistorical is just asking for trouble. They only have so many people and resources to throw at the game, I would much rather they use them to fix and improve the base 1936 start.
If early start times are that popular you would have thought there would be a mod for them. Even Kaisereich doesn't do anything earlier than a 1936 start.