Because, Bitcoin needs to be mined, to buy the POS token.
Note that people can be rewarded in this Proof of Stake network directly, so miners could find it more profitable to mine less on Proof of Work, and validate more on Proof of Stake. It is a matter of incentive, dropping the difficulty from the current level to the bare minimum 2^32 hashes is technically possible.
Not something "environmental evangelists" like LegendaryK are going to support of.
Why not? Using the difficulty equal to one will be "good enough". Going even lower is also possible, because it is possible to increase block time, for example from 10 minutes to 1 hour or longer.
So... Why don't you buy bitcoin?
I do. And I have plans with staking, be ready for them.