1. Because thats what I want and thats really all that matters.
2. I've owned a couple of lined fretless over the years(Ibanez ATK and Squier VM Jazz) and two without lines(Corvette Standard and an old Kramer PJ), as well as spent a lot of time on upright. I also currently own an NS Design NXT upright. There is a different mindset that happens with that big open fretboard. For me, the mind goes to a more bass guitar place when the lines are present, but without them I subconsciously end up in a different creative mindset.
3. I feel that the lines present a false sense of security. Just like the creative part of it above, things will be more in tune if you use your ears and play music vs see line and it must be right. It's for this reason that I also would shy away from an unlined fretboard that has little markers at every single note. That part isn't a total deal breaker, but I would prefer to only have the marker on the side at the 3,5,7,9,12 and so on. Either that or something like what NS does with their instruments.
I have a whole list of other things that I do and do not like. I was really just trying to get people to chime in on what is out there and available since searching the online retailers is only turning up a few things, most of which have been mentioned.