When you apply to be cabin crew airlines expect you to meet rigorous health standards because you'll be responsible for passengers safety and comfort. That means good eyesight (with or without glasses) normal hearing stable blood pressure, healthy lungs and heart and overall physical stamina.
You also need the strength and agility to move quickly in the cabin, handle heavy luggage if needed and respond effectively in emergencies. Any medical issue that slows you down like poor vision hearing problems heart or lung illness asthma uncontrolled blood pressure can endanger both you and the passengers.
Safety-critical environment: In a plane cabin crew must be alert to emergencies: first aid evacuations helping sick passengers sudden pressure changes etc. If your health can't support that reliably airlines consider it too risky.
Strict regulatory standards: Many rules (national and international) demand that cabin crew have no chronic or severe medical conditions or issues that might get worse or interfere with duties such as heart disease, severe asthma neurological disorders etc.
Long hours, irregular schedules: Cabin crew often work long flights multiple time zones jet lag cabin pressure changes. That requires high stamina and resilience. Even minor health issues can worsen under such stress.
Reliability and consistency: Airlines need to ensure their crew will perform consistently every flight. If someone has a medical condition that may flare up say allergies asthma hearing problems they may be judged unfit to avoid unpredictable risks.
If you want to avoid rejection due to fitness/medical issues take care of your health seriously long before applying. Regular checkups good diet fitness routine and honest medical history are essential. Also make sure vision and hearing are correct with glasses or lenses if required and stay free from chronic issues like asthma uncontrolled blood pressure or heart/lung problems.