Because the older you get the more you realise that nothing actually changes when you go from e.g. 19>20, you feel exactly the same as you did the day before and it all sometimes rather anti-climatic.
When you were younger a birthday was a day where a bunch of things that never normally happen, happened. An entire day was about you, you received that present you have wanted for ages, all of your friends are there and you went out to somewhere really fun and got to eat all the crap you wanted. When your older and actually have your own money, going out and getting treated is less of a novelty, as a student you will probably just go out and get drunk, which is what you would have done anyway so nothing special there.
t;dr your birthday is just another day, feels less special.