Youth Thunder, sorry about late reply, I don't visit this forum often.
Why all Buddhas and arahants don't teach? I think because some of them can't, Pacceka Buddhas (silent Buddhas) for example. I think this is where 'silent' comes from. Initially the Lord Buddha, the one we are learning from now, also didn't want to teach because he saw that not many will fully understand this Dhamma, so this may be one of the reasons too.
Arahants - my Ven. teacher says that not all arahants are the same. Although having an arahant state of mind, they still have their human characteristics and traits - for example, some may be outgoing, some quiet, some may have very lively sense of humour, some may be very serious at all times, some may chose to live within a community, some prefer solitary life... also, some may have the inclination to teach, and some don't, similar as non-enlightened beings. Again, if we look in the scriptures, there were many arahats living in the time of the Lord Buddha, but not all of them were teaching. There are many different factors which will determine if someone will teach or not.
On the other hand, we could find someone who is capable of teaching quite well, but has not reached any attainments and doesn't even practice what he/she is teaching, like Tuccho Pothila - Venerable "Empty Scripture" (empty book).