“DOS applications were rarely well behaved in terms of using existing system resources since they typically ran one at time. Developers often used tricks or even existing bugs in the OS or processors in clever ways to get more memory or prevent running of unlicensed copies. Windows was intended to enable multiple applications to run at the same time, so it was difficult to support running existing applications while isolating them so their clever tricks would not corrupt other applications.”
— Tandy Trower [technologizer.com]
And that was 25 years ago. Ouch.