Children consume a great deal of time and energy. Even after they're bathed and put to bed, cleaning and preparing for the next day eat into "adult" time in the night. Even splitting a perfect 50/50 with your partner (if you have one) doesn't leave much time for personal projects.

Having a house also means spending time on its care. Not nearly as much as children, but it's a non negligible amount.

Marriage can be less of a barrier to personal projects. Just depending your relationship and schedule.