Guidelines are, by their nature, random. Some people become addicted to alcohol after their very first drink. Others drink way over the limit for years with little obvious effect.
When Shane MacGowan released FairyTale of New York few would have given him more than 5 years - He's still here , Kirsty isn't. Many other music celebs also drink/did drink very very heavily. For some it killed them, others are still around.
This doesn't mean that everyone is fine drinking 20 units a week. Some cannot tolerate any, others tolerate a lot more.
Less than 14 units is considered 'low risk', (it's also lower risk if this is spread over at least 3 days) more is considered 'higher risk'. In 1979, less than 59 units was considered low risk for men (women weren't even relevant). SOME of the risks also relate to behaviour under the influence (male (generally) violence) rather than direct medical effects.
Every country has advised different advisories. Genetics play a huge part in how much alcohol your body will tolerate/process etc.
My family (blood relations) all drink more than the recommended amounts, as did my parents. My parents both lived (and I mean lived) into well into their 90s. The majority of my generation is now healthy and in their 70s.
Too much alcohol increases risks - it doesn't necessarily mean it will be an issue for any specific individual - just that higher intake increases risks. It's up to each individual to weigh up the odds and make their own decisions.
If you have alcohol problems in your family then you are at higher risk if you drink at all. If you are a family of above average intake but with no (obvious) associated health issues, I would posset it is less likely. But who knows. We are all individuals.
I drink more than advised. I'm currently healthy, live alone with no dependants so I'm ok with my intake. Others weigh up your own risks.
One thing is sure. It's not the case that 14 units is fine and 15 will kill you