Companies intentionally make their web experiences worse because they can track you better in apps. Even if the trackers are technically the same, even somewhat savvy people using blockers or browser protections generally don't use them on their phones.
This is a satisfying narrative but I feel like you are overestimating the technical competency and/or malice of these companies. The more likely explanation is that they have simply bitten off more complexity than they can chew.
Of the tech companies I've worked for, I can't even imagine how the web team would react if they were instructed to intentionally nerf their website.
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Old Reddit has the advantage of being pretty much static non-interactive content. No video, tiny thumbnails, and barely any JS or styling. Some people like this and some don't, but the end result is a very lean website that performs well out of the box.
https://x.com/jimsimon_/status/1841087335414280571
Suffice to say, I'm on the frontend perf team and we're acutely aware of these problems
https://x.com/jimsimon_/status/1841092341991403974
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This was in October 2024.
Which is of course a bunch of bullshit when you consider that Reddit's backend returns most data in under 400ms, and it takes Reddit frontend 3+ seconds to render it
It could be that they are just incompetent.