So do other riders. And helmets may be required by law in your area. A bicycle helmet reduces the peak energy in a sharp impact. This requires a layer of stiff foam to cushion the blow by crushing. Once crushed, the foam does not recover.
Spongy foam is added inside for comfort and fit.
So it needs a strong strap and an equally strong fastener that cannot be jiggled open. The helmet should sit level on your head and cover as much as possible. Above all, with the strap fastened you should not be able to get the helmet off your head by any combination of pulling or twisting. If it comes off or slips enough to leave large areas of your head unprotected, adjust the straps again or try another helmet. Keep the strap comfortably snug when riding.