Because part of their job, if they are good at it, is to present some data, measure how customers are responding to it, and then tweaking said data to get a better response. If you make it too hard for them to “tweak” it, then they will be more reluctant to tweak it. Thereby doing a poorer job.

Them being able to easily change the website is critical for them to be able to do their job. As in most things in life, it is a balance that needs to be achieved and not one thing that trumps everything else.

As others have said, WP gives them a way to easily change the data that they are responsible for. It should be the devs/systems engineers responsibility to make that data present as a static web page using plugins or caching layers and that should be transparent to the marketers.