Why do I get the urge to do 3-4 side projects just when I don't have time for neither of them and when I had time to do them I was just doing nothing all day...

I get that too

So that you can continue doing nothing all day while cheating both the thing you're supposed to do and the side projects.

Living with the same..
That said you should feel good about it. Nothing against anyone as there are good reasons to it but have seen devs working monotonous 9-5 job and be happy with it.
I cannot see it happening for me. So many things to learn and we aware of!

But trick is still to balance it and include in your workflow. Harder said than done!

Some would say that is ADHD or ADD.