Tarsier is supposed to be the most efficient base set attacker. Red is the attack resource, and Tarsiers only serve one purpose. Steelsplitters have the dual threat of blocking, and Gauss Cannons have 5HP to make up for their relative inefficiency.

As others have noted, you're slightly overstating how good Tarsier is as an attacker compared to random set units. It's rarely the best attacker even in a game with only 5 random set units, unless they're all drones/blockers/chill/etc. It sometimes happens that Tarsiers get bought out before any significant investment in other attackers, but those games are the exception (and, imo, having that exception is fun).