In most cases these traces are shipped over the wire to a vendor. Only that will cost $$. Then, not all vendors have tail sampling as a "free" feature. So, in many cases it's better to not record at all.
I do not do cloud or web development, so this is just totally alien. I generate multi-gigabyte logs with billions of events for just seconds of execution and get to slice them however I want when doing performance analysis. The inability to even process your own logs seems crazy.