Not all cultures put the same weight of importance on birthdays the way that many Western cultures do.
And in many Muslim countries, the celebration of birthdays is considered a heresy at worst and just a thing for children — and not adults — to celebrate at best.
“Is that really true? People have known the day of the year since prehistoric times…”
Knowing your birthday and actually caring about it are two different things. And caring enough about it to actually remember it is something else entirely. In many cultures, birthdays are only really acknowledged as children’s holiday to celebrate and not just a thing in general for adults to celebrate.
If you are not raised in a culture that truly reinforces your birthday being something worth remembering and celebrating, you might just as well end up forgetting it.
But that is the celebration aspect of all of this. As far as basic knowledge and why it might be “forgotten” over time, the Wikipedia entry for “Birthday” is quite useful. This is from the section on Islam and birthdays:
“Some Muslim especially from Salafi school of thought oppose the celebration of a birthday as a sin, as it is considered an "innovation" of the faith, or bi'dah while other clerics have issued statements saying that the celebration of a birthday is permissible.”
“Some Muslims migrating to the United States adopt the custom of celebrating birthdays, especially for children, but others resist.”
While Salafi adherents are effectively conservative Muslims, still it seems they are not innovating an idea of birthdays not being important but rather amplifying a cultural norm of being ambivalent about birthdays.
Additionally, many countries like Saudi Arabia, have had extremists actively going around and post posters that explicitly forbid any birthday celebrations because “…this is heresy under Islam.” as explained in this 1999 BBC News article; bold emphasis is mine:
“Posters have been going up in Saudi Arabia condemning the celebration of birthdays and anniversaries.”
“According to the religious decree now circulating in the capital Riyadh, it is not permitted to hold parties or exchange presents on the occasion of a person's birthday because this is heresy under Islam.”
And while this is an overall implication on my part, one can easily see a war torn Muslim country like Afghanistan creating an atmosphere where birthdays are not as important as other things… Like not dying.
“One In Three Afghans Lacks Identity Documents”
In addition, as explained in this November 2017 article shared in the comments by @user3067860; again the bold emphasis is mine:
“Afghanistan’s government estimates that as many as a third of the country’s population -- an estimated 10 million citizens -- have no identity documents.”
“Afghanistan’s current population is estimated to be 30 million. These figures are based on census surveys conducted four decades ago. Most Afghans now carry a handwritten ID card called Tazkara. It is a vital identity and legal document.”
10 million out of 30 million is a massive amount people who are not accounted for on a basic level. But the zinger is in the last sentence:
“The current and previous Afghan governments have failed to conduct censuses because of insecurity, a lack of resources, and political wrangling.”
What we consider something incredibly basic — a very basic form of identification — is a logistical and political headache in Afghanistan. It’s a fairly good bet that many of those 10 million people who did not have basic, written ID cards don’t feel they need one either.
If they don’t feel they need even a basic handwritten ID, chance are good they don’t care about a formal, full birthday as well.
Thus someone not remembering their literal birthday because, for the bulk of the life they have already lived, it was never a thing to begin with at best. At worst, it was a thing that — if shared — would put them at risk to the whims and actions of others in their community.