HFT aren't the firms putting the markets on computers. HFTs sit between the computers and the retail traders and use that information advantage to front run.

There's no reason there needs to be a middleman here. Buyers can buy from sellers directly. In fact that's what they think they are doing, except that the HFT firms are basically adjusting the price on them underneath the sheets.

The real losers are the sellers. They put up some product at a price, someone else on the other side of town sees a slightly different price because the market only moves at the speed of light. Instead of the seller getting a little bonus, that bonus is hoovered up by the HFT guy in the middle who has a more accurate view of the market.

What service did the HFT provide? Where did this mysterious liquidity come from?

In the old days the middlemen were responsible for actually finding the guy making the sale and matching him up with the buyer. This was real work and it makes sense that people would be paid for it. Now the computer does it for you, there is no need for a middleman and no value he can offer. It's a parasitic relationship.