I guess manufacturers assumed people will first and foremost use them to move data around, not to backup/archive it, which makes sense as they were somewhat pricey initially, while the role of "write once, read many" was occupied by CD-R discs.

Of course, at some point the prices dropped, and eventually some people ended up owning a dozen USB sticks of various sizes and shapes, some of which used for archiving, but none of them labeled.