As a frame challenge to your question, I'm not sure that it makes sense to take a (potentially limited) description of one person's PTO availability and extrapolate it to an entire nation's workforce.
I'll use myself as an example. In my current role, I have 26 days of PTO, 12 paid holidays, 5 paid sick days, and 3 days of paid volunteer time (I'm able to take time off work as long as I do volunteer work for a non-profit on those days). At a prior employer, I had 18 paid holidays, 36 days of PTO, and 5 sick days. I have a life to live and value availability of time off, so when I'm selecting jobs, I'm sure to evaluate employers on that basis. Others may feel differently, and may target employers who compensate in other ways, versus time off (advancement, or salary, or other things).
If we assume your theory is true, and there are (on average) less days of PTO in the US, your question of "why" may have many different answers. While there is less regulation in the US than in some countries around paid time off, this isn't inherently a good or a bad thing, as it essentially means that PTO becomes another point of negotiation when working out the details of a job offer. Some people see this as an advantage, since it means they can push for what they want, rather than having their outcomes dictated by regulation. Other people want to be protected from potentially abusive employers and would rather work under a more highly regulated environment. Unions play a role as well, when they're able to act in a manner where they can collectively bargain with an employer to set certain standards for things like PTO.
Further, Workers in the US also have protections in terms of being allowed to take time off for specific circumstances - for instance, if you have to take leave to care for a sick family member, FMLA protects your job under certain circumstances. And we have programs where people out on disability or for other reasons either have protection for their jobs or may receive (limited) pay while they're out. As such, these special cases don't need to be accounted for when an employer determines how much PTO to offer.