Because you get issues with the most common last names in pretty much any language. Do you realize how many people out there have the same Last name? And does this system take into account homophones? Because if not, you didn't actually gain any meaningful benefit out of this effort.

It would result in December babies being named 'Xyoabxoanz Smith" or similar.

Edit: just looked it up and did some napkin math. There were 2,369,644 Smiths in the USA alone in 2020. Half of them are women, and lets say about one third of them are in the age where they can have children, which leaves us with around 400k female Smiths in a childbearing age range. US fertility is at around 54 children / 1000 women / year, meaning you would have to assign around 21500 unique first names every year. This is before taking homophones to Smith into account, which will make the issue far worse. The user who said Unix Epoch would be his choice has a better system than this one could ever be.