It's not because dad is not competent. No, he's able to do all the things the kids require, even if he has to use the trial and error method to figure out what seems obvious to mom.
It's not because dad doesn't really want to spend time with the kids. He loves them and will enjoy time with them - and they'll love that special time, too.
It's not because they won't have fun. They'll have fun - and mom may come home to a disaster of Fun Mess.
It's not even because mom might return to a chorus of little comedians telling Dad Jokes (though this might be worthy of consideration).
The reason dads should never be asked to babysit has nothing to do with dads. It's simply because it's the wrong word. When the kids are left in the care of dad, it's called parenting.
It's babysitting when you ask someone else to look after your children, who doesn't have the inherent responsibility of caring for them that parents have.
This was drawn to my attention by my friend Janet over a dozen years ago when my eldest was a baby. In response to my comment that my husband was babysitting, she quipped, "That's not babysitting; that's parenting." And I've never forgotten!
Of course he's not babysitting! It seems so obvious when you say it like that. When dad walks out the door to work in the morning, no one thinks mom is just babysitting during the day until he comes home. Parenting is a team sport. They both took on the responsibility when they welcomed the children into existence within that family setting. It's what makes the whole group of them a family.
There are a great many things that are much, much more difficult for single parents (whether they are widowed, separated from a spouse or unmarried) than for families where both parents are present together. These parents can, of course, do a fantastic job of raising their children, but it takes a lot of support. It's not the ideal.
When the family is intact (mother and father both in the home raising the kids) it's still hard to do some things. If they both see the job of parenting as theirs together, not just the mom's, they will work together to see that the children are cared for and each parent is also able to do the things that help keep them functioning well.
When parents see the job of parenting as primarily mom's - dad being an occasional "babysitter" - mom's needs can be overlooked a bit. Some things are just harder to do with a baby or several kids in tow. How does a mom get real exercise, have prayer time, read a book, maintain friendships give blood or a myriad of other pursuits necessary to personal growth if she is literally never alone?
One small thing we can all do to support families is to just stop saying dad is "babysitting" when he's alone with the kids!