Why does an object traveling in a circle at a constant speed always accelerate?

Because "acceleration" doesn't mean "speeding up". It means any change

in the speed or direction of motion.

An airplane speeding up, a bicycle slowing down, and a car going around a curve, are all doing accelerated motion.

A circle has no straight parts, so an object traveling in a circle is always changing

its direction. That means accelerated motion, even if its speed doesn't change ...

I'm not sure but I believe it is momentum, when an object keeps moving and keeps Its speed