Why would something people find fun enough to do as a hobby now not have been fun 500 years ago?

Compare it to say a modern profession like software engineering. Despite it being work, there's plenty of programmers who also enjoy programming and do it for fun on their own time as well as work.

Why would it have been any different back then?

There are plenty of people who don't enjoy programming. Now imagine they all have to do it too.

We consider programming work. Overwhelming majority of it is pure work.

Today's labor is different in that the individual is commoditized. You're indentured to your client, who then accrues a debt to be discharged in an agreed upon time with various contingencies appended, like showing up on time and regardless of completion of your given task (10 baskets/8h) you're nonetheless expected to put in your contracted time.

Basket weaving done in your home, with performance left to your own scruples, and a personalized schedule is leagues different than slaving away for someone else.

A remarkable attempt to redefine one out of work, but in both cases your activity is means to an end (of survival).

Subsistence farming is not leisure.