Because they have NOT been solidly refuted (disclaimer I am a Professional Computer Scientist with a CompSci degree ).
An operating system should be like the old Volvos, gradually developed and constantly improved over time.
And the Mac's OS X is a certified UNIX operating system, sharing the same lineage as Oracle (Sun) Solaris, Open/Free BSD. In short it builds on over 30 years of solid experience with many users and deployed on many different types of machines (this is why it was easy for the Mac to jump from Power PC to Intel).
All MS products are developed in a closed bubble, with Windows 7 only being able to trace it's code base back to Windows NT (early 90s).
While its true that no Operating System should be blindly trusted to be secure, it is exceptionally easy for Windows systems to be hijacked, due to the poor design, lack of outside review, and general culture and attitude of Microsoft.
While you will hear about vulnerabilities of Mac OS X from time to time, they rarely if ever go beyond compromising a single user's account. MS will constantly blame 3rd party software (i.e. Adobe) for these kind of problems, but this just underscores MS incompetence, since its the job of an Operating System guarantee that bad programs don't allow bad things to happen.
Even when an MS system is bogged down with 3rd party anti-virus software, Intel hardware hacks ( NX bit ), hardware and software file walls in place, they still get compromised on a regular basis.
Any unbiased observer would have to admit that a company like MS with the vast amount of resources available to it, would have greatly minimized this problem if it really wanted to, especially since there have been many other systems which have solved this problem decades before.
By showing the architectural reasons for a relative lack of exploits in the wild, you are helping in the refutation. They think the only reason effective viruses aren't in the wild for OS X is because of low marketshare, making it not interesting to the malware writers. They forget that while OS X has 50+ million users, people wrote highly successful malware for pre-OS X MacOS, and for other product populations with numbers well under a million.
As far as "first to be hacked," the reference is to hacks that took weeks to develop in advance, which were then released in the second round of a hacking contest to win (Safari was the culprit all three times). Windows machines were hacked on-premises. All it showed was that Macs are more desirable than PCs.