I guess maybe a better question would be "why do some Christians seem to think they have monopoly on the concept of heaven?"
Christianity isn't the only religion that has an afterlife concept of some sort of bliss, ascension or closeness to deity. Some Christians seem to think they have rights over who gets in though...as if it is an exclusive country club...or at least they'd like it to be. Goodness knows these self-righteous Christians would hate to think they've bowed and cowered and thought themselves better all their lives for something that someone who did none of that gets as well. Kind of a kick in the shins huh?
Your absolutely right, Christianity does not have the only afterlife concept, is'nt it interesting to note that many others have some sort of eternal life concept linked to there religion. Which one will you buy into, it's like the biggest gamble one will make, ..or is it. Can you know for certain.
Heaven is exclusive and the heaven Christians talk about is the heaven the bible talks about and while it is open to all to come to that heaven there is only one way a person gets there..that would be "Jesus" any other way would be the false way.
You may believe in your version of God and/or heaven and that everybody go's there one way or another and maybe you don't believe in hell.
The question I ask you is are you absolutely, positively sure ....or is this an intellectual assumption for arguement sake only.
Have you experienced this heaven here on earth, do you have an internal excitement and joy that your going there or an assurance and certainity that it is your destiny.
If you don't.... it's possible you have either a myth or a vivid imagination