"An additional objective for this flight will be attempting an in-space burn using a single Raptor engine, further demonstrating the capabilities required to conduct a ship deorbit burn prior to orbital missions."
The feasibility of building big rockets was already demonstrated a long time ago. Given the reliability of the Falcon 9, it looked plausible that a big rocket could work with many engines. And SpaceX had already shown that they can reuse boosters economically. But reusing orbital spacecraft – the entire upper stage with engines, fuel tanks, and whatever – without expensive and time-consuming refits is something nobody has done before.
SpaceX can launch satellites using Falcon9 and do it routinely. Starship needs to be developed and reach milestones, so they can get paid by NASA. Having a payload is a complication (unless it's a fun payload, remember the roadster car :D)
Yeah you might lose the payload, but SpaceX has the cheapest satellites in the business from what I understand.