This is not really true of course. There is some series of physical and chemical causes of why some given DNA sequence is the way it is. It’s ancestor used to have some sequence, and then there is some physical cause of a mutation in that sequence, giving us the descendant sequence, same would be true for the ancestor, which itself has physical causes of why it is the way it is, all the way back to the origin of the first genetic sequence.
You might not be able to name a physical “law” as being responsible, but it is false to say it isn’t due to physics and chemistry(there is no physical “law” that explains why yesterday’s weather was the way it was, but physics and chemistry does explain it, through the interactions between the Earth’s atmosphere with the sun and so on).
There is definitely a complex history of physics and chemistry that explains both any extant, and any ancestral DNA sequence. The DNA sequence having a contingent and complex physical history does not mean they have no physical and chemical explanation. That’s a non-sequitur.
That would be like saying there is no physical explanation for the particular arrangement of matter we call the Mt. Everest. We might give a generalized explanation by invoking things like gravity, plate tectonics, and electromagnetism, as all being parts of the explanation for the particular arrangement, but these forces must then be understood in the context of the initial conditions of the solar system and the formation of the planet and so on, and of course we can go back further still to the initial conditions of the universe.
You might say there’s nothing in the force of gravity alone that demands the existence of the Mt. Everest, but if we think about it we can still see that gravity is still very much a part of the explanation for the Mt. Everest. The pressures exerted by the gravitational pull of the planet are part of the explanation for it’s particular size, shape and structure. Then there are all the electrostatic interactions between all the atoms that make it up, and how pressures in the Earth’s crust and mantle have changed over time etc. etc. Thus there really is, in principle, a total physical and chemical explanation for the Mt. Everest, it’s just practically outside our ability to provide. It’s not that physics and chemistry can’t and doesn’t explain it, it’s that we just lack sufficiently detailed knowledge of the system that gave rise to it to give this explanation.
All the same things are true for any given DNA sequence. There really is a physical and chemical explanation for why some particular sequence exists. It’s just much too complicated and too sensitive to local conditions at the atomic level at any given moment in time for us to be able to give a complete world-history for all the subatomic particles that are part of that explanation.