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

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

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 ...