From what I can tell the short answer is that you're right, there's really no good technical reason for all that weight. Which is to say, it's just bloat.
The JS performance difference between high and low-end devices is stunning (9s vs 32s load times, based on the Medium article). Web devs, who are often used to using the latest and greatest devices, will have no idea how terrible their code performs on slower devices. And I fear that modern CPUs with excellent JS performance will only exacerbate this issue
I had always been using 2-3 year old android phones for the last 7 years and I was unable to comprehend how anyone uses the web on mobile because everything was so incredibly slow in the browser while native apps were fine. Then I got the latest iphone and my mind was blown at how I could scroll a website at 60fps. I guess if you have the fastest phone on the market, everything seems fine.
It's strange as poor performance is known to be off-putting to users, which presumably translates into less traffic and thereby less profit for the site.
Much has been written on this. Here's an article from 2018. (I realise the irony in that it's hosted on Medium.) https://medium.com/@addyosmani/the-cost-of-javascript-in-201...