Why does bitcoin core need to download the blockchain
Certainly miners, and many other users, keep track of the block chain.

But if you don't download and verify the block chain yourself, what will you do when you want to know whether a transaction is valid?  You could ask a miner, but what if they decide to lie to you?  You would have to trust them to tell you the truth.

Part of the Bitcoin security model is that you don't have to trust anybody.  When you verify the block chain for yourself, nobody can trick you into believing that an invalid transaction is valid, or vice versa.