There are many reasons for this.
It could be that your site isn’t optimised for conversion, looks unprofessional or is slow to load.
It could be that your website is ranking well but for irrelevant keywords that cause people to land on your site and then leave when they aren’t finding what they want.
It could be that your website offers a poor quality user experience or the page gives little information about your product, service or business.
Perhaps your site is lacking a unique value proposition that adequately separates you from your competition or you aren’t shouting about the benefits of your product enough.
Basically, there are lots of reasons why visitors may not be purchasing your product or service. Investigating the issue is a tedious task and takes time. If you aren’t willing or able to do it, it means you need to hire someone who is qualified to do it instead.