Because our universe would have been created by a transcendent consciousness (not comprised of the physical universe itself) if deism is true.
This is a logical non-sequitur. "transcendent" and "consciousness" are terms relative to human experience.
The Deism hypothesis adds zero information to "The Big Bang".
It is merely an ontological choice to say, "The Big Bang" / "Noumenon" = god(s).
This implies that consciousness rather than matter is fundamental and is therefore in direct opposition to atheism.
Nobody except nobody believes "matter is fundamental". Noumenon is fundamental.
In genetics, scientists have coined the term "junk DNA" for strands of DNA they believed had no functional role in sustaining the organism because it had been left over from evolving.
Terms like "chaos" "randomness" "dark energy" and "junk DNA" are merely scientific placeholder terms that mean "we have no flipping clue".
If a deity created the universe, this is an avenue for life to have been designed.
This is another logical non-sequitur. We have no idea if the human concept of "designed" either has or does not have any conceivable corollary to something like the Noumenon. This hypothesis is beyond our epistemological limits. And would seem to be a consequence of the anthropomorphic fallacy.
Rather than mindlessly compiled, DNA can be viewed from a design-first approach. And as it turns out, less and less DNA is discovered to be "junk."
Identifying complex patterns and or previously unidentified uses and or "purposes" of "junk DNA" does nothing to "prove" "design".
Such a development would be exactly like the hundreds of years of scientific progress where we discover knowledge that was previously unknown.
Before Democritus, people thought the weather was "fundamentally unpredictable" and when Democritus was able to predict the weather with some modicum of accuracy, the people began to worship him as a god. To his credit, he did his best to explain to everyone that he was just a normal human being, just like the rest of them.
Deism is functionally identical to atheism.