The real question is why isn't tax paid on all capital gains already. It is income from owning an asset.
If I work for $100,000 a year for 10 years and earn $1 million, I pay income tax on that.
If I have a business and sell enough to make $1 million in profit, I pay tax on $1 million.
If I buy a house for $500,000, and 10 years later sell it for $1,500,000, why should I not pay tax on that $1 million profit?
The government isn't planning on taking all the capital gains you get from selling your house, just a fair percentage.