There are a number of potential reasons why your Google PageSpeed score is lower than you expect:
1) Your website might not be optimized for mobile devices. Google's PageSpeed Insights tool uses a mobile-first approach, so if your website isn't designed with mobile users in mind, your score is likely to be lower.
2) Your website might be loading large files or images which are slowing down the page. Try to optimize your images and files to make them smaller, and your score will improve.
3) Your website might have a lot of code that isn't being used. This "dead code" can make your website slower and affect your PageSpeed score. You can use a tool like Google's PageSpeed Insights to find and remove unused code.
4) You might be using outdated technologies that aren't as fast as newer ones. For example, if your website uses an old version of the PHP programming language, it will be slower than a website that uses the latest version. By upgrading to the latest technologies, you can speed up your website and improve your PageSpeed score.
There are a number of other potential reasons why your PageSpeed score is lower than you expect. If you're not sure what's causing the problem, you can try using Google's PageSpeed Insights tool to get more detailed information.