I'm not entirely sure why this is necessarily harder for the mother. I can understand that a mother has more of a connection to a young child, as she gave birth and many mothers desire to stay at home and nurture their children. But dads aren't exactly incapable of these feelings; in fact, I think dads feel the same in a lot of ways. Sure, society has spent many, many years (centuries) teaching us that men provide, but it doesn't mean that we don't feel like we should be at home with our kids. Then again, I am a dad, not a mom, so I may not understand completely. I preferred that one of us stayed home with the kids until they were school-aged, and whether it was my wife or I depended on who had more income, steady hours at work and the like. It just happened to be me in our case.
I hate to think that people think a child under a certain age needs to be "productive." Kids should be able to be kids and have time to just be a kid. Cramming their schedules with a bunch of activities seems to squash that. Maybe one or two things are ok, like t-ball or dance or music lessons, but to have them doing stuff all the time seems unfair.
I think it makes it hard for the child to develop the ability to entertain themselves and be creative on their own. My 6 year old spends hours creating his own stories with action figures and toy cars and he didn't do any extra curricular activities until he hit first grade. And my wife is a stay at home mother, so that makes a difference, but even mothers and fathers who both work can find venues, such as child care, that allows kids to play freely.