Why wouldn't we assume that? Even with the ability to synthesize full DNA molecules, we haven't yet figured out how to make a living cell from scratch. If our best synthetic biologists still find it impossible to assemble a viable cell while having all the tools to do so in theory, I wouldn't expect it to be likely to happen by accident, barring some quantum weirdness that makes the starseeder hypothesis seem positively banal.
As to deliberateness... I see the difference between starseeder and panspermia hypotheses that in the former; life happened once in a gravity well, became multicellular, figured out a means to escape the gravity well, and then spread throughout the galaxy while in the latter; life happened once in space, drifted a while, got stuck in a gravity well and then figured out how to be multicellular.
I suppose - particularly given the vastness of the places with no gravity wells compared to places with gravity wells - that it's my own gravity-well bias that makes me reach for 'starseeder' hypotheses before panspermia ones. But I think whether life was multicellular before or after the bulk of the interstellar spreading, the instinct of life to spread would be the motive force whatever its problem solving capabilities.
AIUI most panspermia theories don't posit either of those - definitely not that life originated in space, and not necessarily that it only originated once. The most common variants seem to be that life originates in gravity wells, is ejected by major impact events, drifts through space and eventually re-enters other gravity wells. Simulations suggest that bacteria can potentially survive all of these.
Why would we assume that? We've been seeing biologically interesting chemicals appear from much simpler ones for well over a century.
https://en.wikipedia.org/wiki/Abiogenesis#Chemical_synthesis...
Also, your "starseeder hypothesis" sounds a lot more deliberate than panspermia theories, but correct me if I'm misunderstanding you there.