Because you only have 2 parents...
You can get one from mum, one from dad and one for where you were born I suppose.
Unless you buy citizenship of somewhere else...
Eh ... I have an Aussie passport even though I wasn't born here and I have no Aussie ancestors. Just like a few million other immigrants to Oz! And no I didn't buy citizenship!
By the way, in some countries you can qualify for citizenship through your grandparents and in some cases where you are born automatically qualifies you for two passports - eg people born in Northern Ireland are entitled to both Irish and British citizenship. The same is true of people born anywhere on the island of Ireland prior to 1949.
So in theory, people could easily be entitled to more than 3 passports. I'm entitled to 3 without relying on where my parents were born (Australian, Irish and British), although I've never taken up the British one. If each of my parents came from other countries, I'd be entitled to 5 different passports!