The new Ibis DV9 is a surprisingly affordable carbon hardtail

I believe the newest giant hardtails can be converted to SS. I think they all, or most, have sliding drop outs.

Consider it from the manufacturers point of view. SS is a very small fraction of sales and to do a bike compatible with single speed means that you need special design elements that can potential compromise the function of the frame for normal derailleur use. Adjustable position dropouts that are compatible with SS and gears are heavier, more complex, less aesthetically pleasing than a dropout dedicated specifically to one drive train type. Eccentric BBs are heavier and have a higher risk of creaking or being misaligned from the factory. Obviously it’s possible to make SS specific frames, but when it comes to carbon that proposition is very difficult to support from the business side of things as those frames will require their own special molds (big $$$), testing, design time, and R&D. When that frame will only sell in very small numbers compared to a geared frame it becomes really hard for the profit on those special frames to pay for the mold costs and design time required. To do special aluminum or steel frame is a lot easier as they don’t require molds, but they still add extra SKU’s, extra testing, extra inventory overhead, etc.
Basically bike companies know that there is a small number of people out there who definitely want and love SS bikes, but more often than not the business case just isn’t there. That’s where specialty brands and niche builders come into play to fill the gaps left by the larger companies.

It’s not as elegant, but using a chain tensioner like the Shimano Alfine works very well for converting any bike to singlespeed. An added bonus is that if you cut the chain right you can even run it “dinglespeed” very easily without having to make extra adjustments between gears.

Assumptions tend to prove false. Agree there aren’t a lot of ss’s out there I do notice just as many carbon frame as steel