Genome encodes functional and dysfunctional information not only in its main chapters of linear sequences and primary chemical modifications such as methylations, also in its dynamic multi-dimensional structures, organizations, functional derivatives in forms of aberrant orders, binding and interactions, loops and folds, locations and shapes.
For twenty years, genomics has progressed to be good at answering one question: " what does this person's inherited genome say? "
That question has a simpler structure. One genome, shared by every cell, in theory. Averaging across millions of cells is the correct method. Deep short- and long-read sequencing plus mapping answers it with remarkable approximation and falling cost, and the informatics built on top of it is some of the best engineering in modern biology.
Somatic disease is a different question.
A tumor is not one genome. It is thousands of genomes, diverging in real time — aneuploid, poly-aneuploid, restructured by chromothripsis and chromoplexy, carrying extrachromosomal DNA that segregates without a centromere and varies from zero to hundreds of copies between neighboring cells. Aneuploidy is a hallmark of cancer, present in up to 90% of solid tumors. Poly-aneuploid cancer cells drive therapy resistance and progression. ecDNA is reported in a large fraction of the most aggressive cancers, including glioblastoma and small cell lung cancer. In that setting, averaging stops being compression and becomes destruction.
In germline genomics, the consensus is the answer. In somatic genomics, the consensus is a genome that no cell actually has.
This is not a failure of sequencing. It's a mismatch between an input and a question. Standard preparation — mixing, sonication, fragmentation, stripping of structural context — dissolves exactly the information that defines a somatic genome: which pieces were attached to which, what was circular, what belonged to the same strand, compartment, nucleus, cell. No amount of depth recovers it, because the information was gone before the instrument saw the sample.