Well, first, that's not true. There are plenty of third party anti-virus utilities that will protect you from any exploit in the wild for the Macintosh. There are even entirely free ones. So you are flush with options.
Second, even if you have an older Mac, and even if it has a potential vulnerability that isn't patched, it's highly unlikely that you are going to see an exploit for that vulnerability.
Here’s the thing. Modern malware is almost exclusively written for financial gain. (With the odd bit of malware written to target a particular socio-political group, usually in the far east. These exploits usually aren’t seen in the west.) Whether it is to serve up ads, or to scam users out of their money, it is all about a profit motive.
In addition, modern malware tends to take a significant amount of time and money to write. The Mac isn’t easy to write malware for, and when a potential vulnerability in the Mac is found, the bad guys have to strike as quickly as possible before it is patched. But “striking quickly” usually still means that it will take months to push out a new exploit, representing a large investment in time and money.
Also, since it apparently has proven to be exceedingly difficult to write actual viruses (i.e. self-propagating/diseminating malware) for the Mac, any malware written for the Mac will almost certainly be a Trojan Horse that will be very difficult to disseminate to a large audience before it is discovered and shut down.
So, the bad guys are looking for potential vulnerabilities that Apple doesn’t know about, which are likely to go unpatched for many months into the future, they want any exploit that they write to have the maximum number of potential victims, and they want to be able to reach as many of those victims as possible, as quickly as possible. This is all a difficult feat.
Presumably, even if older versions of the Mac OS are just as vulnerable to a newly discovered potential vulnerability in the Macintosh as newer Macs are, once the majority of newer Macs have been patched, it will become uneconomical for the bad guys to target this vulnerability. By the time that the bad guys are able to push an exploit out, there will be way too few potential targets left to infect to be able to recoup the investment of time and money they put into creating the exploit.
At least that's the way that it has tended to work out in the past. Owners of old Macs haven’t been beset by unpatched-against malware. Estimates of the numbers of users still using older versions of the Macintosh OS tend to show that there are surprisingly few users of versions that are so old that they no longer receive any security updates from Apple.
Global macOS version market share 2018-2021 | Statista
So, older Macs by themselves simply aren’t a viable target for malware writers. And existing malware that can no longer effectively target recent Macs tends not to remain in the wild because it can’t self-replicate/disseminate, so it isn’t a significant threat to older Macs.
If it does occur that there is a bit of malware in the wild that is patched in newer Macs, but which is still going around infecting older Macs (and this has been the case years ago), it’s extremely likely that someone in the Macintosh community will come up with a free patch (which, once again, was the case in the past).
As long as Apple remains fairly diligent about patching against security vulnerabilities in the most recent versions of the Mac OS, the entire Macintosh community should remain safe due to a sort of “herd immunity” effect.
Well, best of luck to you finding an operating system and computing platform better than the Macintosh. I'm sure that we will all miss you. Please write now and then and tell us all about how much better your (different) computer is. I'm sure that we would all be fascinated to hear about what that better computer is.