Imagine how for every one user doing this deliberately there are nine who pause a video and forget it in the background, wasting bandwidth in the process.
You showed your home bandwidth. It means absolutely nothing in this discussion.
How often do people watch the first few seconds of a video and not finish it? It happens a lot. It probably happens a lot more often than that user actually finishing it. We could be talking about doubling Google's bandwidth requirement. Not to mention server CPU time, disk I/O. Do you have any idea how expensive the operational costs of YouTube probably are as it is? This is an efficiency game to successfully run a video platform which supports up to high bitrate 4k video at this unfathomable scale, servicing the entire planet.
It makes the most efficient sense for them to only let you buffer a little bit at a time, not more than you need.
I'm not kissing Google's ass. I'm just pointing out that if you want the service to exists, it has to be designed as efficiently as possible, otherwise it won't exist for long.
Like others mentioned - yes, I mean the bandwidth from the perspective of the one providing the service. For the same bandwidth that someone watched 10% of a video, paused it and never watched the remaining 90%, you can show those same 90% to someone else who'd actually watch it. That's without counting the small overheads here and there, but hopefully you get the idea.