Ok, PDF readers are developed by people who think oppositely to what user wants
We UXers would be so happy if there was "what user wants". The truth is that different users want different things.
It so happens that you prefer a workflow for which continuous mode is more convenient. This is not universal! For example, my preferred workflow with PDF documents is to size the PDF reader window to roughly A4 size and then flip each page as if I were reading a book, using the arrow key on the keyboard. For this reason, I never use the continuous setting, as I find scrolling through a PDF disorienting.
As you can see, the UI which best supports these two different workflows is very different. So there is no way to make a program whose default settings were good for everyone. Indeed, if some feature has a perfect setting for everyone, it is not made a setting, but hardcoded.
You just happened to use a PDF reader whose authors choose as a default the setting which supports the "other" user group better, the users whose workflow is different from yours. They also gave you a simple solution: you can go to the preferences screen and change it. (I hope that they allow you to change the default as opposed to requiring you to change it again for each document, as that would mean that they did do a bad job after all). This is what I do all the time, because funnily, the PDF readers I use are somehow always set to continuous mode out of the box.