Many higher end preamps are dated and ugly, why is this?

Yeah, lame topic but so what? Schiit gear works very well for my tastes regardless of how it looks (my Freya looks cool though), and don’t let women with no pants sit on your carpet...trust me. 

Pretty is in the eye of the beholder. There’s ’art’ that sells for multiple millions that I don’t think pretty.

’Outdated’ ... well ... maybe after some 100 years of evolution in analog signal amplification circuitry it has come to a somewhat optimal design, from where it’s very hard to improve on it any further, or to find a whole new, better, design principle.

Technological progress has brought us transistors, IC’s, and nowadays even powerful digital processing on a chip. Much more sound shaping is possible in the digital realm and this is widely used in the sound studios where the digital master recordings are created. Yet, for play back many listeners still prefer the 'ancient' analog (discrete component) (tube) technology.

All a matter of personal taste.

CJ was dated when I first said no to my sound advice salesman in the early 90s. He didn’t understand. Fortunately the Krell KSA -100S came out soon thereafter (I did with Adcom until then). Using McIntosh Hybrid these days and it looks the part. The brand was too staid for me back then. Wadia, Rowland, Acurus, Gryphon, Levinson have done it for me as well as Krell and Mc. Schiit is just too lacking in style and size to do anything towards a purchase. I’ve looked, but won’t listen. 

now I am making (outsourcing to several companies) an exclusive enclosure for the power system.
Bronze , bone , exotic wood ... development at the jeweler , foundry , CNC milling ...
It turned out to be quite expensive! - the mass market, on the contrary, is trying to save money (for this reason, the external design is very modest)

My Zesto Leto is sexy as hell,  but I bought it because of the rear panel