C++ Programs and GUIs

[Vindiesel walker]

It seems there are an immense number of GUI libraries for C++ 
which caused me to raise a few questions.

Why are there so many in the first place? (Obviously they offer 
some advantage over one-another or, in theory, the best one would 
be the only one.)

Can I write a command line program and then add a GUI later? (How 
much code would that involve re-writing?)

Which libraries/toolkits are common in professional program 
development? (Or do they just write their own libraries from 
scratch?)

What do YOU use to add a GUI to your programs?