The reason why Windows OS computers are “virus susceptible” is b/c they are/were far and away the most computers and OS on the planet for the last forever. I’ve been reading that w/ the increased popularity of Mac devices, they are being targeted by viruses on an ever increasing basis.
Thus, Macs were largely virus free (until lately) as the scammers and mischievous makers wouldn’t waste their time on such a small percentage of PC users worldwide. They want the big bang for their bucks when phishing or just generally writing a nuisance program for fun. Plus, their programs get promulgated far faster via MSFT Outlook than the Apple email program.
Unix, upon which Mac is based and which Linux clones, had a better fundamental security model than Windows until recently. Windows required you, in essence, to be root all the time if you wanted to do anything useful with this all too accommodating schmoo of an office operating system that wasn’t even engineered for network usage. Windows 7 finally toughened things up, but I think even it still can get viruses.