Why Is Heaven Free?

Here's what the Bible has to say about it:

"For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God— not by works, so that no one can boast." Ephesians 2:8-9

There is that word again - GIFT. It is clear that heaven is truly a free gift of God. But why can't we earn it?