There were a number of interventions accessible by the government. As parent points out there was ideological resistance to employing them, and where employed they were done so weakly and without enthusiasm. Sir Charles Trevelyan used the same argument you do - that there was ample actions Irish landlords could take directly - as an excuse for the government not to take action. Given he described the famine as a Godly judgement on the Irish, that argument is not a reasonable explanation for his approach.