Because applications on the web are increasingly that -- full applications.
I create scientific models and simulations for use in schools. Whether it's simulating a hurricane, or continental drift, electronics, or molecular interactions, the simulations themselves need to run on the browser, and all the UI that provides the users with all the affordances to interact with the model needs to also be written in JS/TS.
I think your questions are just revealing a failure of imagination/experience for what kinds of applications run on the web these days.
> it's no longer the web is it - just a way to deliver traditional applications
This statement is meaningless to me. What makes it "no longer the web?"
"The web" now includes fully-fledged applications. It's fine to make a distinction between things that are full applications and things that are close to blogs, if you like, but it doesn't change the fact that many people develop full applications for the web.
And I think this is clearly a lot more common you are recognizing.
The fact you ask this question gets at my point I think.
For several years I've been writing a large computer algebra system(CAS) that runs on a webpage. Every time the user puts some input into a text box the CAS runs. Depending on the input it may run as many as ~40k lines of code. There are no coherent lines upon which to split the CAS as far as anyone developing it can tell.
The CAS must run on the browser both to deliver on real time performance requirements and to keep server costs manageable (certain inputs will get even high end CPUs humming).
If breaking this SPA up is possible, it's not apparent even to engineers with >10 years of experience developing highly complex applications.
Other similarly complex applications run on the web, even if it's unusual.
The trick is to not break it up. Adding more lose ends will just make the ball of twine more complicated. What you can break out though is independent code that is the same for many apps, that code can be made into a module and reused across apps. Pure mathematical functions can be turned into a library.
But what could probably help keep your sanity for a CAS is to add a test case for every change to make sure the same input produce the same output in the future. As well as performance tests to avoid performance regressions.
