I use bookmarks for stuff I may want to access later (as in another day), not stuff I want to access within a few hours. If I used bookmarks for everything my bookmarks would be even more inflated and unorganized than they currently are. I'm not one of those people who keeps hundreds of tabs open for weeks on end, I just have a habit of opening tens of them each day. I usually close my tabs once or twice a day anyway.
Also if you have a low amount of RAM (which there are some that still do), you may not want to only have 2 tabs open at once. Sometimes it's much more convenient to just leave stuff open.
You do have a bit of a point though. I could start using bookmarks instead of pinned tabs for some things, and I guess a good bit of it boils down to me being unorganized.