One general reason is because ear plugs they are wearing are not convenient like this one below. Look at that ear plug. I am not interested to wearing it too. It can easily drop off while being used. It may not be able to reduce high noise level since it depends how we wear it.
Hearing impaired can be occurred in the workplace due to high noise level that continuously exposed without using any hearing protection. Hearing protection like ear muffs and ear plugs could reduce high noise level to a safer level. But, sometimes people feel not convenient wearing ear plugs due to various reasons.