I wouldn't say that HFT arbs out only the "obvious" things. We try to arb the non obvious things as well, with more or less success.

HFT just means you trade a lot and you have low tick to trade latency. It doesn't say much about why you are trading.

For example you could have a HFT stat arb strategy in which you are arbing higher dimensional factors that are pretty far from obvious.

In general, the speed wars are over and everyone is reaching deep to become as smart as possible. Or in the other words, just being fast is no longer an edge, it's a commodity.