The reason is partly because the oceans are slow to warm up compared to land,   but that's not the whole story.

If we were to warm earth's oceans by 1℃ 1℃  and then stop (in our real world "experiment" we haven't stopped yet), the land would warm by about 1.5℃ 1.5℃  and then stay warmer than the oceans, even after the ocean has 'caught up' to the land. The land and ocean surface temperatures have a similar relationship in the warming and cooling of year-to-year variability.