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

If the manufacturers sell out their production run, why would they change their design?

Pretty is as pretty does! 

Or

Amps got to sound good to look good and the amp looks marvelous. 

The Anthem STR although solid state, is not pretty, but pretty damn sexy. 

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’Outdated’ ...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.