As a web developer, my job is constantly changing, usually getting more complicated. The skills you need to build a fast and easy to use content managed website today compared to 10 years ago are completely different, and if you don't adapt and keep learning it's easy to fall behind.
PHP is the language Craft is built on (actually it's built on Yii 2, a PHP framework) and although I feel like perhaps I should know more about how it runs under the hood, the reality is that if I don't use something regularly, I'm going to forget it. I've done several PHP courses in the past and I remember almost nothing because I never have cause to use it regularly. It's also more of a backend language and I generally find front end work a lot more satisfying.