- not having to update due to issues in dependencies of dependencies of dependencies
- not having to rewrite it all because framework maintainer decided to shift to a new paradigm and no longer maintain the old version
- not having your massive framework generating weird request flow that isn't defined in any RFC and can't integrate with industry standards
- not having to pay for specialised hosting because only one or two companies know how to properly host and scale a website built with your massive framework
- being able to integrate or understand actual web technologies because you don't have a massive framework abstracting it all from you
Some frameworks or libraries may have been designed with some of this in mind. If you want to scale and integrate with other stuff, pick wisely because your whole code base will end-up tied to that framework, especially if "full-stack". Tho I have to admit developper experience can be awesome.