I guess there is different kinds of 'busy'
Busy 1: The 'I have a life which is sufficiently occupied by tasks that I am content/have taken on too much' regardless of what the tasks are out what sort of percentage of time they occupy.
And
Busy 2: my life is very full from non optional things like working so I can pay bills, caring duties and basic household chores.
A person in category 2 will often have very limited leisure time, hobbies etc, not rest enough etc.
Whilst I can't see why a person in category 1 might feel busy, they are often have tasks which they could stop doing at any time. No one of forcing them to go out and have fun and they could stop without being made homeless or neglecting their children etc.
I can see why a person in category 2 would feel frustrated, when, for example, working full time and being a single parent to children, someone complains that they are too busy because they like to do gardening and get their nails done a lot.
My in laws often expect us to take time off work to cover and fix things like computer problems because they are too busy to drive the laptop to us (retired, no voluntary activities) even though we have two small children and both work.
They may feel busier than us, but it's not them getting 6hrs sleep on a good night, and with little time for fun...
Honestly, I've been busier in life. I wouldn't quite put myself fully in category 2 right now (but have been) so this isn't me feeling bitter. I love that people can fill their lives with fun activities, and be 'busy' with fun.
However, just as I wouldn't complain about long hours when working 20 a week to a to fellow parent who works 60, telling a person who is knackered through non optional tasks that you are busy with fun isn't going to get a lot of sympathy surely.