CK2 is probably affected by the fact that its earliest starts are tied to expansions, I'm pretty sure its post-1066 starts are equally unpopular to EU4 post 1444 starts. The general rule for our games is that almost everyone will start at whatever the earliest date they can play on is.
Bang goes my Alexiad start in CK3 then...
For EU5, it'd be really nice to have a handful of choices. No need to have every day of every year, but at least 5 starts?
EU has a couple that I've played a lot, Start of the Dutch revolt, and the rise of brandenburg when still a vassal of Poland.
Even if avid players don't always use it, the choice to do so is really appreciated...