Bears repeating.
This really isn't true; there are possible observations that would lead to a design inference. EG:
On one of the pulled threads (IIRC) a few weeks ago, one of the creationist/id-ers was going on and on about genetically-engineered hogs and sheep that contain human and spider genes, respectively. If this sort of thing were commonly observed in the natural world, (especially the interphylum gene transfer), *and* viruses were pretty much as they are now (they can do lateral gene transfers, but it's quite rare), then some sort of genetic engineeering would be a reasonable explanation.
Or, along similar lines, if genetic data didn't arrange itself into trees, or different genes made dfferent trees, or the genetic tree didn't match the already-known phylogeny, it would be reasonable to assume something other than common descent, mutation and selection was at work.
However, the overwhelming majority of genetic data very neatly fits with other genetic data and the already-known (or suspected) phylogeny.
To quote my favorite example for the N720-th time, where N is large: If a psuedogene, transposon, etc, is found (in the same place) in the genome of both chimps and orangutangs, it will also be found (in the same place) in gorillas and people. This sort of *fact* points to a common ancestor, not design (which is in principle arbitrary in the absence of any limits on the hypothetical designer)
Don't blame the biologists for being close-minded; blame the data