Bitcoin has a market cap of 70 billion. This is very low when compared with the stock market or the gold market. There are many groups that can buy the total number of Bitcoin in circulation with a price of $4000. They can easily manipulate the market both sides up or down. Bitcoin 70% supply is stored in a very less number of wallets say 5%. Don't you think Bitcoin can easily manipulate and below $1000 is still possible?
Your title is misleading. Bitcoin is not easy to be manipulate, since if it would be then that would be really bad money. But as you are explaining you dont really talk of Bitcoin but Bitcoin price.
If anyone would want to buy all Bitcoins that have owners. that should be around 9 millions then price would increase so much that there is no such wealth on earth.