A lot of other commands require parameters that the attacker may not know the values for. Those require more knowledge about the system that first needs to be exfiltrated. Not every (hacked) query has its output directly plastered back in the UI, webpage or API response. So that would need more hacking, even if the "sleep" tells the hacker they're on to something. Disabling "sleep" would take away the canary, would it not? Maybe the hacker isn't even interested in dropping tables.