Whenever someone says "it can't be that hard" I have to wonder how much they actually know about programming/game development to make that claim. Don't get me wrong, at the end of the day I would like time to be slowed down as well. But since I am a programmer I also know it's not as simple as changing one number and being done with it. When you have stuff that relies on a timer, in this case how fast days go by, there's a high chance you have to rebalance everything around that new time. Slowing time means they'd have to readjust how fast animals get hungry/thirsty, how fast facilities break down, how fast staff works, etc.
So you should never really say it would be easy to change, because in game development, nothing is ever easy. I just wish we could get an official response on everyone wanting the time slowed down.
They're using the same game engine as Planet Coaster, albeit a newer and better version, and that game had 10sec days vs PZ's 3sec days. There wouldn't be anything programming or engine wise stopping them from making the change, other than it going against their vision for the game.