WordPress has become, by far, the single most used content management system on the planet. It has been around for over 10 years, with numerous fixes for each of its issues. That being said, let's check out a few of the things that makes WordPress the best CMS currently available and why most engineer teams be they freelancers or in-house developers, recommend that you use it.
Any site can be hacked, it doesn't matter if it is custom PHP, Wordpress, Drupal, Joomla, Prestashop and so on. It is the developer's responsibility to design a secure website. The American government uses Wordpress as its go to platform for a lot of their public awareness websites. It features a lot of safety measures that make it literally impossible to hack. Unlike custom PHP which will always have flaws because of its "custom" nature.
No framework or CMS is "SEO friendly". Some are EASIER to setup than others, but at the end of the day it's up to the developer to make sure you have proper h1, h2, etc tags. Meta tags, alt tags, unique content, etc etc. WordPress has tons of plugins that make your job A LOT easier in that regard.
Wordpress is very easy to back-up; should anything happen, it is a matter of minutes until you restore it to its full working capacity.
Sooner or later something will come up and you will want it integrated into your website. With a custom PHP framework, the development time, costs and end-results will take very long to come into fruition. With Wordpress it is only a matter of minutes or hours, depending on the difficulty. Never months.