My Nimbus 20" arrived and when I tried to test it, the tire was pretty much flat to the ground, as in there was like no air in the tire. Does UDC sell their Uni’s with low air so the rider can inflate it to their liking? There is also no PSI guide on the tire, so not sure what the recommended is for a uni.
I am 5’7", 200lbs.
They come deflated or very low psi so in case of the contents overheating during shipping, the tube won’t explode. Don’t go by a number, such as 25psi, 30, 32, whatever. Adjust the pressure so that you can compress the tire enough to get a good bounce, but not so low that the tire/tube bottoms out on the rim wall. Also depends on what type of riding you’ll be doing. For road riding, flatland and freestyle, people usually like higher psi for the lowest rolling resistance, while lower psi is usually used for trials and MUni.
I use 20psi in a 2.5 trials tyre / 40 in a 1.95 freestyle 20" / but was happy to use 45-50psi in the 20" when learning from scratch because it felt right.
Air is for sissies. I put concrete in my tires.
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Required air pressure also depends on what you’re doing - I currently have ~35psi in my 2.5 trials tyre as I’m working on stuff which involves rolling and that makes it less sluggish. 40-45psi is likely to be good for you as you’re a bit heavier than me.
My last unicycle from UDC came without the tire or the FOSS inner tube installed, so consider yourself lucky! 
Most tires will come with a max psi rating on the tire. As long as you don’t go over the max psi you should be fine. You also don’t want it so low that the rim hits the ground or whatever your’re riding over.
Generally, the lower the psi, the smoother the ride, but you’ll have more resistance and have to pedal harder. The higher the psi, the less resistance you’ll have, and the easier it will be to pedal, but it will be bumpier.
Aside from the extremes mentioned above, there is no hard and fast rule like a car tire. It’s just your own personal preference. Get a decent bicycle pump (with a built-in psi gauge if you like) so that you can adjust it as needed. There’s been a lot of discussion about tire pressure in these forums, and you can use the search function at the top to find them and get more insight.
Enjoy your new uni!
To answer your question; I think they ship unis with uninflated/underinflated tires so that they can fit them in a smaller box.