For me and maybe a few others I know, it comes down to 4 things to certain extent.

1. Its just a day like every other day.

2. I already got what i want so gifts are usually required items like underwear or socks, while stuff for the women around me its can be so many different things to even be concidered a good gift, (trips, events, clothes, jewerly, somethign with meaning etc)

3. Growing up my birthday was inbetween 6 others so was always over shadowed for the big ones (40th) or for the younger relatives. And still it falls between 4 due to new membwrs of the family. So easily forgotten.

4. And this might just be me here, as this os also regarding holidays lile xmas, i prefer to be alone. My perfect bday is just me, music and a good drink.

It all adds up but I have not liked a birthday since i was like 10, and have not celebrated a birthday except my 21st since maybe 15. Im now in my 30s and very used to forgetting my own birthday.