Because what most websites, ecommerce stores need per any given view/page are a few unique, isolated jQuery functions to manipulate what is strictly necessary. Be it listing a category listing of products, be it adding to cart by clicking a button, be it updating quantities in cart, address or payment.

The 'modern' frontend frameworks take way too much after frameworks like React that were born from social networks in the inception of the social network age a decade ago. Facebook needed to have people poke each other, like posts and comment under them, while at the same time incrementing like, poke counters, as well as listing and updating a crap ton of friends, page and group listings on the sidebars, notifications and inboxes at the top and in the bottom bars and a whole lot of other stuff.

So while, say, for example React solved a major problem with there not being a major templating system or logic in the front end up until then, it also brought it with the baggage of the mentality which assumes that we need that kind of dom manipulation at any given time. True, one can indeed use something like React and keep it minimal like in the shopping cart example above. But it rarely happens so and instead even the business logic starts seeping into the front end.

The time when social networks exploded and such extensive DOM manipulation became 'cool' as a result, was a time in which the frontend was stuck in between Flash and the emerging jQuery/JS mess that some preferred instead of Flash to make websites 'modern/cool'. It was 'professional' for sites and apps to interact with users back in the early internet. Flash was used for it, then it became uncool as the web moved to jQuery, JS etc. Social networks exploded right in the middle of this transition, amplifying this trend. You wanted a 'modern' website that had moving parts. Not a plain HTML + simple CSS + JS website even if it loaded fast. Every widget and form had to be active, interactive and do stuff. Facebook was !all! the rage in that period, and everyone literally imitated them in everything they do, including tech stack and practices. Then Twitter also amplified the trend. Everything added up on it, and we ended up with the frontend mess where we tend to shove everything and then complain about complexity...

Thats not what modern frameworks are evolving to, though. Svelte doesn't have a "touch everything" approach like React. Interactions are compiled at build time and only the JS needed to change the specific things that aren't static is generated. There is no runtime.

With jQuery on the other hand you're bundling a whole bunch of stuff you probably don't need to do things modern vanilla JS is perfectly capable onlf. And you'll have to reinvent the wheel every time either way.

Sure, your simple approach is probably enough for small stores selling a few items, but it really doesn't scale to all of them. And it makes for a way better development experience to use a modern framework for all but the very simplest of interactions.