I have not seen any quantitative data on this, so just going of my own anecdotal evidence. I think there is a couple of aspects to this. One, beginners tend pull way too much with their arms and that tends to tire you out very quickly (your arms are pretty small muscles compared to your legs). It also requires some experience on the ergo to learn that you can make hard or soft pulls at the same stroke rate (again this is difficult if you pull mainly with the arms), so beginners tend to just adjust stroke rate. The other aspect is that rowing produces lactate in pretty high rates because of so many muscles being involved. That takes a bit of getting used to.