Because the malware writers will keep tweaking the code until it passes that check, just like virus writers submit their viruses to VirusTotal until they are undetected.
its Typical that the Virus Writer will use their own service, there is criminal virustotal-clones that run many AV in VM and return the Results, because virustotal will share all binaries, anything upload in Virustotal will be detteceted shortly if it is not.
Because the malware writers will keep tweaking the code until it passes that check, just like virus writers submit their viruses to VirusTotal until they are undetected.