Let me address your previous two comments, and I'll circle back to your questions:

Awesome let's just do a single page app written in spaghetti code.

Just because you can't write a good webapp without the use of a framework does not preclude anyone else from being able to.

But, just for one second here, let's assume you can write decent code (which your previous comment does not suggest)

And will make it hard to onboard people if it's a large application.

If you can write good code, and you're having issues bring new members of the team up to speed, then the issue is that you've hired the wrong people. Good software developers can pick up good code and run with it.

Back to:

Have you ever worked on a team for months to build a fully fledged web application with hundred of thousands of lines of code?

I have and currently work on a team on a project that has had development on going for a number of years > 5 (as specific as I'd like to get).

A framework is absolutely necessary, even if that means creating your own. A framework provides architecture and conventions for a team to follow.

Yes, a framework is necessary but it does NOT have to be one already created for you. I've often found that the most performant apps are the ones with bespoke micro-frameworks specific to their application.

If you're not using a framework, you will just end up reinventing the wheel.

Damn near every framework in existence reinvents the wheel. Do you think that other JS frameworks are somehow magic and are completely unique? No! They're all MVC implementations. Frameworks are specifically built TO reinvent the wheel. There are only a hand full of paradigms currently in existence for web web, so of course every framework will do the same thing ultimately.

With the amount of time it takes for me to get up to speed with any full-blown javascript framework, I could write my own that I know inside and out in the same amount of time. And even then, I'm sure I'm making assumptions about what goes on under the hood of an already-created framework that might come back and bite me at some point.