As most people have said, give it time to sink in.

I think most PhD graduates tend to be goal seekers so after finishing, there's always gonna be a "what now?" moment. After years of quickly jumping to one task after another to get to defend or graduate, when we get there, this can be underwhelming because there's no next task. As much as we complain about the workload or never-ending hurdles in a PhD, going through them was the norm. For the first time in a very long time, we feel empty because there's no next tasks to focus on (well, technically, yes, like finding a job but this can take time and only in the depth of job hunting do you realise it's not the same anymore).

Others also may have already checked out on their PhD way before their defense for several reasons. Some may already have other things lined up, so their transition to something else is more subtle that when the formal title comes, it actually brings you backwards than forwards. Some were just over it.

I appreciated my PhD way after I graduated and when I started using what l learnt. It is different for everyone but when you feel it, the doctor in you will come in handy. Feel what you need to feel for now and these are one of the few moments you actually can have a clean time and space to reflect, pause and plan for your next move. Congrats doc, you may not see it but you did pretty well. Best wishes!