Well...unless a stepdad, or all stepdads tell you it's easier, how can you really say being a stepmum IS easier.
Easier, is subjective as well. Many stepdads take on the financial burden of stepkids and the mum stays home with no job....while another man supports her kids.
I find stepmums seem to be more jealous, complain more, see themselves as better than the mum and are more controlling over a lot, which just causes them stress.
There is so much insecurity with second wife syndrome and the regret of not experiencing the 'firsts' that many (not all) stepmums have. There seems to be this fear that he'll go back to her, or the Ex wants him back. Women need to be wiser in choosing a man who isn't a lazy so and so looking for a replacement mum for his kids.
Women assume caregiving responsibilities, even without the societal expectations and then it backfires, when they have enough of being taken advantage of. We can't ignore the fact some single dads just want a woman to help them out, or parent their kids. Women fall for this...I'm not sure if it's out of a bit of desperation.. then they have so much regret, that the Ex is ever present.