As someone who develops web apps for a living...probably all the shitty websites / apps out there.

Product doesn't care about optimizations, they care about speed to market / customer. That means I also have to not care, so all the code is janky AF and full of nice O(n^2) or worse algos and keeping shit tons (hundreds...thousands...tens of thousands...) of full 100+ data point payloads in memory even though the app only needs one tiny field from it. etc. etc.

Also all that telemetry. Yum.

I recently learned my friend works at a company that has telemetry in their web app that screenshots the page and sends back what the user is doing every 3 seconds or something. As well as the current state of the app. It lets developers rewind/replay situations for debugging. But the sheer amount of data captured and sent over the wire blew my mind.

edit: Sorry ELI5...lots of sites are suboptimal in design and hold on to or track a lot more than is actually needed for proper functionality. Couple that with telemetry that either Google or the web app itself wants to gather on everything you do and you're in for a bad time.