Hardly surprising that it would have less functionality than device costing 8 times as much.With a volume control it would be a preamplifier as well. That is indeed how I use my RME ADI-2 DAC, straight into my power amplifier, and it works very well.
Would think integrated amplifier more common than power so possibly wider market for it.
Just seems you are faulting a horse for not being a zebra.