why is inflict moderate wounds a 3rd level spell and inflict serious wounds a 4th level spell but inflict light wounds is a 1st level spell?
The same reason the paladin has cure light wounds as 1st level, cure moderate wounds as 3rd level, and [i]cure serious wounds[/] as a 4th level spell. Because that is how the designers wanted it.
As to why that is true, you would have to ask Wizards of the Coast. They are the ones that did it. Paizo merely copied it, then gave the antipaladin the inflict versions at the same level.
Druids get Cure Light at 1st, Cure Moderate at 3rd, Cure Serious at 4th, and Cure Critical at 5th.
Rangers get Cure Light at 2nd, Cure Moderate at 3rd, and Cure Serious at 4th.
Witches get Cure Light at 1st, Cure Moderate at 2nd, Cure Serious at 4th, and Cure Critical at 5th.
Not all classes follow the same pattern of learning healing spells that the Cleric does.