To my opinion it's a combination of conditions

1. Open standards. There is no one entity to control what frameworks are "standard". Need for back compatibility of the web standards make them too simple and minimalistic. So you inevitably have a demand for power tools.

2. Many players are trying to capture demand for power tools. Of course their reasoning and approach differ. Often it becomes less about technology, but more about audience perception of the tool. Then we see more blogposts, podcasts, meetings, "see how easy to create and app" posts.

3. Web frontend is hard environment to model UX because too many moving parts. You have inherentedly stateful programm which should communicate asynchronously with at least 2 inputs-outputs: web server and user. Not speaking that it's not enough to have framework with proper building blocks to model your UX, you have bunch of issues with dependencies, debugging and integrations with existing tools.

This makes hard to come up with a solution which can satisfy many people. Some people are infiltrated with ideas which are incompatible with particular framework ideology, and bam, you have 2 camps which "push" against each other.

Now you have demand for tools, but tools shitty in some ways, and it's shitiness is covered with hype, brands and other forms of forming opinion.

This creates a demand for a new thing after people get disillusioned via own experience. And we get yet another framework. Repeat every 4-5 years.

JS front-end frameworks are born from ad hoc application architecture or from difficulty applying an existing framework to an application. The progenitor applications had different feature and development requirements, which is reflected in the varying frameworks.

While it may seem a plethora of frameworks exist, a few have most of the market share. That is not too far from the situation with other programming languages and framework types.

There are also old frameworks that have essentially been obsoleted but continue to stick around due to project inertia, the needs of legacy code, or marketing needs of the maintaining organizations.

Some people prefer FP. Some prefer OOP. Some need just a sprinkling of interactivity. Some need to build complex UIs. Some need to run on underpowered devices while others don’t. Some dislike too much abstraction. Some prefer it.

In short, we have a plethora of frameworks for the same reason we have a plethora of languages, sodas, beers, coffees, tools, products, novels, etc. Humans are messy.

"Can someone explain to me why we have so many programming languages?"

Different strokes for different folks. People are trying to make new things that solve what they think are pain points in previous approaches.

There really isnt "so many" frontend frameworks. React is the clear leader, with Vue and Angular trailing behind. Svelte is a bit more of an experimental/innovative thing.

There is quite a long tail of small/niche frameworks, but for most folks there aren't many major ones, really it's 3; React/Angular/Vue. There's probably another 5-10 in the next cohort, including Svelte