Explain why Newton's first law of motion can be difficult to observe in moving objects?

That's because in our daily life in the real world, nobody has EVER seen an object that's in constant uniform motion without any net force acting on it.

EVERY object we've ever seen has gravity acting on it, buoyancy due to the air surrounding it, plus friction and air resistance if it's moving.

To all of our senses and observations, the First Law of Motion should be:

"An object in motion, with no force applied to keep it going, stops."

Except that's wrong.

possibly because most object don't stay moving forever like If you roll a ball across a table then a force causes it slow down and stop called friction.