[justus_]

As you may know, programmers are notoriously bad at building anything and most applications are riddled with holes. There's so much you can get wrong, especially if your app is meant to communicate over the internet and be accessible from the outside. Not just crypto, though that is the most prominent one but Middleware, form protection, upload handling, traffic monitoring, authentication and caching without blowing up the RAM, etc.

I also don't think your app would get more scalable. You might have less code overall in your app but how well documented is it? Is it as well documented as the frameworks? For whom one of the most important concerns is usability and making sure others understand the code? Overall I doubt your code would be very readable. Many people write awful code and I doubt there's a lot of in depth code review in most companies, requires too many resources.

Another point when it comes to scalability is interfaces. Interfaces to talk to other instances, interfaces for pluggable modules, interfaces for the database, interfaces to different frontends. If you want your app to be scalable and possibly distributed and/or extensible you need good interfaces and programmers are also notoriously bad at designing those. (That's why JavaScript is the way it is) The only reason why those of frameworks are somewhat workable, is because they made a lot of mistakes before and learned from it (mostly). Good interfaces require constant and incremental improvement. A maintenance effort hardly any company would like to bear.

[shellbye]

Such a great answer.