Fritzing offers many different microcontroller boards in breadboard view and schematic view.
It would be impossible to make a simulator which covers all these different boards sufficiently !
And: there pop up several new such boards every year ...
If you add complicated periphery to your microcontroller you have to check it in realtime anyhow.
You could not trust a simulation result -- there are always some deviations and surprizes
If you use only transistors and some analog ICs you can use SpiceSimulators form different semiconductor
companies. I'm using this in order to build e.g. some special filter circuits that are added to the analog input
of the microcontroller board later.
However, in any case one needs suited measurement equipment (scope, etc.) to verify if the ideas work ok.
Have fun, rgds, Uli