Depending on the Project they absolutely will. Consider the economic context this is happening in.
There is a staggering number of companies that can harden and secure a WordPress or Drupal Installation. They have scalable workflows that allow them to keep the attack surfaces small, keep up to date with any security issues and even deploy countermeasures before any fix is released.
The market is just big enough for those services to exist. Now consider the same for a custom built Symfony or Laravel CMS.
You still have to trust Third-party code that has had security issues in the past, one just assumes that one crowd is more trustworthy than the others.(http://symfony.com/blog/category/security-advisories)
Besides that, the number of companies that are able to harden, maintain and host is much, much smaller, because their workflows can only scale so much.
So your gains are not really on the security side of things, rather on the implementation flexibility. Which CAN mean a much more Resource Intensive Project.
Drupal and WordPress both have their birthdefects which have ingrained so far into the community that you can't expect them to be ever worked out fully. This leads one to your conclusion that because they aren't doing some parts of their job particularly good, they shouldn't be considered.
Having worked on large WordPress, Drupal(just when Drupageddon happened) and Symfony Projects I can say that from the viewpoint of a customer, the CMS Win out simply because they make the chain of people working on the project more accountable, comparable and replacable. The risks of being technologiclly dependent on the few people that wrote your custom CMS just arent worth it for the customer. The Risk of Running the same Wordpress Code as millions of others sites is manageable in comparison. With Drupal you can go as far as just calling Acquia if all else fails, their customers were safe on Drupageddon since they could just Filter the malicious requests. You can't expect your Symfony Hoster to do something similar when their day comes.