Why do callable bonds typically have a higher yield to maturity than noncallable bonds, holding all other

Question:

Why do callable bonds typically have a higher yield to than noncallable bonds, holding all other things constant? Is the yield differential between callable and noncallable bonds likely to be constant over time? Why?
Maturity
Maturity is the date on which the life of a transaction or financial instrument ends, after which it must either be renewed, or it will cease to exist. The term is commonly used for deposits, foreign exchange spot, and forward transactions, interest...