Just because things are computerized doesn't mean firms are willing to provide liquidity. The number of natural counter-parties at any one time are low, this is

Just because things are computerized doesn't mean firms are willing to provide liquidity. The number of natural counter-parties at any one time are low, this is where HFT comes in, as market makers. Also if you look at the pricing mechanism of ETFs, they simply couldn't exist without HFT. Arbitrage is essential to ETFs, they simply wouldn't exist without low latency creation and destruction of shares.