An expectation persists that DNA should have been recovered from the car because some victim blood may have been carried into it. This expectation may be driven by the "CSI effect": DNA being thought to persist indefinitely and be almost impossible to completely remove from surfaces. With 7 weeks to repeat clean it is very possible and straightforward to remove all forensically usable DNA from the car.

In studies testing cleaning of DNA from various materials including carpet (similar to car seat material) water alone was sufficient to remove all detectable DNA from non-porous surfaces in most cases, and common household cleaners with "active oxygen" including peroxide stronger than 3% was shown to destroy all DNA on carpet fabric in one wash. Simply washing knife handles with dish wash liquid was shown in a study to fully remove deposited DNA from wooden, metal or ceramic materials.

Various studies have shown peroxide and household cleaning products based on peroxide, or peroxy acids "active oxygen" "Non chlorine bleach" are highly effective at degrading blood and DNA. Hydrogen peroxide is cheap, readily available from pharmacies and supermarkets and costs less than $2 per bottle. Peroxide decomposes to only water and oxygen and leaves no analytically detectable trace or residue. It does not bleach or leave marks on colored fabrics. Peroxide is the "bleach" used in color safe fabric cleaning products. Anyone thinking it's nearly impossible to wash DNA off fabric surfaces might consider that lab cleaning products, even DNA removal "wet wipes" that work in minutes, are sold for use in labs that handle DNA (like forensics /medical testing labs where removing DNA contamination on surfaces is critical) - some of these are pictured above. Forensic examination suites for rape victims, hospital bed linens/ scrubs and similar are routinely sterilised and cleaning procedures are validated by DNA testing to confirm absence of bacteria, viral or human DNA.

Had Kohberger taken simple steps to limit blood carried into his car, such as using a water proof seat cover of a type sold cheaply in supermarkets, removing and bagging a coverall or hoodie & gloves before getting back in the car, blood contamination could have been limited. (As speculation - perhaps the missing shower curtain from his apartment was used to line the bottom of his car trunk?).

There are many murder cases where no victim or suspect/ perpetrator blood or DNA was recovered despite much more challenging circumstances for the killer to clean. A few high profile examples:
Israel Keyes raped and violently murdered Samantha Koenig in a small garden shed - he kept her body there and mutilated it 2 weeks post mortem (including taking pictures with her eyes sewn open), and finally dismembered her. Keyes boasted to the FBI that no DNA or fingerprints would be found and none was recovered.
In the Delphi case of Abigail Williams and Liberty German it appears (based on PCA and court filings so far) that no suspect/ POI DNA was recovered despite the two victims being stabbed to death, their bodies dragged and staged and one re-dressed post mortem at the scene.
Robert Wone was stabbed to death inside a house - despite police sealing the scene within 40 minutes and the coroner stating two-thirds of his blood was spilled at the scene, zero blood or DNA was recovered - the 3 men at the house where he was stabbed appeared freshly showered when police arrived and were charged with obstruction including suspected cleaning of the scene.
Michaela McAreavey was strangled & beaten to death in a hotel room and placed in the bath - despite her body being found within 1 hour no DNA was recovered.
If DNA and blood can be totally cleaned from the scenes of violent, bloody killings in less than one hour, it can be removed from a car where no-one was actually stabbed with 7 weeks to clean.

Cleaning all car surfaces with hydrogen peroxide, pouring or spraying it into crevices/ cracks like the seat belt retractor or foot pedals, would not be difficult, would degrade all DNA and would leave no forensic or visible traces. Given 7 weeks to repeat clean the car, the fact there was no recovered DNA or blood is not hugely surprising.