Exchanges are exchanges and hardware wallets are hardware wallets - they serve different purposes, so you can't compare them directly, they don't compete with each other. People are keeping coins on exchanges not because they view it as a wallet, but because they don't bother to move them out, or maybe they prefer a convenience for trading without making deposits/withdrawals. Also, you should keep in mind that software wallets are more widely used than hardware wallets, so if you want to analyze outflows from exchanges, hardware and software wallets should be viewed together.
It is indeed not reasonable to compare these 2 different entities, as they are serving different purposes.
Hardware wallet is merely for storage of your coins, whereas, exchanges are for trading purposes.
They have totally different purposes so why compare these 2? And a user can buy more than one hardware wallet, just think about it.
I don't see a point of comparing these 2 though. You can't create exchange easily and survive this market.
So definitely, only few have the capability to sustain this kind of business and maintain it thru ups and downs of this volatile market.