I'm working on one of those webforms projects right now, a legacy project that they need some fairly trivial tweaks to and it's an absolute nightmare of bad code. And I've done this before, it's become an accidental skill that I can still fix these awful messes.
One of the (many) reasons why webforms and the new Razor Pages are bad is because the code gets split up according to the UI instead of by function. So it gets scattered all over the place and is incredibly hard to do maintenance work on it.
The page-centric code layout that Webforms/Razor Pages/all the old PHP/Perl/etc. encourage is also extremely conducive in encouraging copy-and-pasta code for programmers trying to get stuff done asap.
So not only is it a nightmare to pick apart the code, you can often fine 2 or 3 copies of the same code that you only discover when one page is working as expected, but the same functionality is used somewhere else but someone just copied the page/control instead of actually splitting the page up into controls and re-using functionality.
I've seen this happen over and over and over. A single developer can avoid these pitfalls, a team cannot.
The height of WebForms coincided with an embrace of web standards and accessibility which flows into the Web 2.0 era. You had to jump through a lot of hoops to achieve what was needed WebForms to get it to behave in a web-friendly way. The underlying .NET framework and base of ASP.NET (HttpHandler and HttpModule) was outstanding though.
(I still build/maintain WinForms, ASP.NET, and WPF apps.)
Who did; which "people"?
Corporations and "Web developers" maybe; users never asked for it AFAIK.
The problems started when you wanted to put together a REAL application. Because the entire paradigm of a web application programmed like a desktop application was flawed it resulted into an endless stream of headaches and workarounds, with bloated, complex and slow applications as a result.
In that sense ASP.NET MVC was a breath of fresh air.
It got better by ASP.Net 3, but MVC/Razor was much, much better imo.
And WebForms wasn't a misunderstanding. It was a deliberate and brilliant design that brought WinForm developers and their experience to the web and allowed them to build complex web apps two decades ago that still work to this day.