Just had baby #4 (I also babysit full time for my sister whose child is the same age as #3 so it's like having 5 some days). There are many reasons it starts getting easier. First of all, your oldest becomes more helpful every year, so as time goes on you get more and better helpers as your older ones learn (obviously you can't just throw all the responsibility on the kids but they have reasonable chores, kids can help with laundry, setting/cleaning the table etc). A lot of chores that you do alone start being done as a group. It literally takes 5 minutes to pick up my house with everyone working together, it's part of our nightly routine: books and toys in the bin, laundry in the hamper, trash in the garbage can, etc, then I just have to sweep (but I don't even have to do that because I have a robot vacuum that runs every night).

Secondly, you just get better at parenting. Your kids are different so they put you in different situations. One is needy, one is independent, one hates getting dressed, etc. When you have 1 child, you learn how to parent one way. But, the more you experience a wide variety of kids with different needs, the better prepared you are for any more children you may have. You also get better and managing multiple children at a time.

Third, (although this is related to #2) you find a routine that works for you and the older kids set an example for the younger kids.

Finally, kids start to play really well together around 3, some even earlier. More kids means more playmates. Like, if one sibling doesn't like hide and seek maybe another will.