The primary activity of HFT and algorithmic traders is fundamentally different. Whereas algos are trying to "beat the market", that is find alpha, HFT is trying to "beat the market makers". They trade dozens of times every second to find the liquidity distribution and arb the flows, not take advantage of technically or fundamentally driven moves.

Needless to say HFT requires several orders of magnitude more resources than a hobbyist algo trader, though there are also giant algos in hedge funds.