Because building a complex WordPress platform depends on intimate familiarity with the CMS and a careful selection of a theme and plugins (if any).
WordPress Site Building
Most vendors either discard WordPress entirely or create a LEGO by bundling dozens and dozens of randomly picked plugins combined with a premium WordPress theme. Both approaches are flawed.
People tend to blame WordPress all the time. I’ve described the pros and cons of the platform in a relevant Quora answer: WordPress is maybe the worst designed and legacy coded (not totally OOP), among popular PHP software out there, but why is it still so demanded in the market?
The reason WordPress is so badmouthed is due to its plugin and theme ecosystem.
WordPress Plugin and Theme Reviews
The plugin review process is extremely loose. There’s a small review theme (comprised of 3 volunteers as far as I know) who run a basic test and a static code review for major flaws. But it’s not extensive and isn’t performed anymore after the plugin is accepted.
Which is why plugin authors can inject almost anything they want. It’s rarely malicious, although that had happened previously. There are some automated scanners checking for malicious scripts but they can’t catch everything.
Also, there are various 3rd party plugins – and many free plugins have upsells to their premium versions. Those are not moderated by the community, either.
The theme review process is great – but that’s limited to WordPress.org themes. Most people buy premium themes from ThemeForest or other marketplaces who accept templates containing tons of 3rd party libraries (and often, embedded plugins). That has a massive impact on performance, stability, and security.
WordPress Development Isn’t LEGO
So yeah, it’s easy to grab a bunch of plugins and bundle them into your WordPress installation. This may very well cause various problems – but that depends entirely on the quality of the plugins you’ve picked.
That’s what I’ve described further in “Don’t call yourself a developer if you don’t code.”
After all, you can integrate any 3rd party system or a random library and risk the stability or security of your website. Blaming WordPress here is a fallacy.
Which is why professional tech companies using WordPress may pick the WordPress core and build almost everything else from scratch. Or pick from a small list of trustworthy, well-developed plugins known for their quality.
This makes complete sense when your platform depends on a CMS itself. Magazines, business listing platforms, training systems, and many more revolve around content management. A real estate listing platform is a good candidate here, too.
Custom Builds vs. WordPress Projects
The more your scope goes beyond a CMS (whereas the WordPress Core incurs some overhead), the more it makes sense to consider a custom build. I personally won’t suggest building a Twitter or an Instagram clone on top of WordPress – there are far too many features integrated into the Core that won’t be used and will impact the load times.
But the vast majority of the platforms out there do need pages, some forms of organized textual content (categorized), admins or editors managing information, media uploads – which are well-tested and proven to work in WordPress.
In some specific cases, the limitation of a generic platform (be it WordPress, Drupal, Joomla) may lead to slower load times. This can be prevented with a custom build, but WordPress provides mature mechanisms for dealing with that (object caching for database queries, image optimization libraries for lossless compression, page caching serving static content through the web server, to name a few).
But the database schema is somewhat generic – and denormalized. If you are to process hundreds of millions of records, a different relational schema may work better. That said, if the rest of the CMS is still helpful, you can easily decouple the content layer and store data in custom database tables – overcoming that limitation once again.
The best and most reputable professional agencies are acquainted with the pros and cons of WordPress (or any other platform they specialize in). They can reuse whatever the CMS provides out of the box and use proven techniques whenever the go-to approach isn’t the most suitable one.