And what if those "cloud based office tools", which would be using proprietary file formats internally, shut down? Or heck, what if they announce to raise their pricing? All of your docs gone. No possibility to migrate.
When the original application is open source, its format is public and anyone who wants to create a converter to another format can either use the original code or inspect it to see how the format works.
When the format is proprietary, you get a blob of binary data that you have to reverse engineer through guesswork, made orders of magnitude more difficult if the original service isn't even available anymore.
PDF is a mess because it's an ancient originally-proprietary format based on a proprietary language and the Adobe "standard" didn't actually specify a lot of important things for a long time, leaving other implementations to try to reverse engineer them.
Meanwhile you look at actual standard formats like PNG or UTF-8 or, as you say, HTML, and the implementations generally do a lot better because there is a proper spec.
So lets have all your data vendor locked in a cloud somewhere so that the corp runnning it can milk you for ever or just decide to delete it all on a whim ? Perfect!
There is an entire generation who have grown up with the cloud and just see that as the default because it is.
Which isn’t to say that everyone in that generation thinks the cloud is the sensible way to do it or that prior generations didn’t (it wouldn’t have “won”).
My boy only understands what a file system is because I explained it to him, all his tech (with the exception of his gaming PC) might as well be magic black boxes for the amount of understanding he has on how it works.
That's the whole problem from my point of view - we have a whole generation that don't really know anything other than these centralized magic black boxes.
That's very dangerous, as it gives a lot of easy to missuse power to the few people actually running these centralized black box services & their victims will not be able to do anything about it due to lack of knowledge how things actually work & viable alternatives.
You will be relieved to learn it is also good. I have used it for years without issue - standard format for libreoffice / openoffice. It is a much better thought out document standard than Microsoft's abomination.