It is not hard to understand why.
Workbooks are relatively inexpensive.
They are logically incremental and modular, usually with one or two pages devoted to a narrow topic. Adults learn paper and pencil computational processes and symbol manipulation on routine repetitive problems. These problems are then followed by real-life applications or word problems whose reason for being seems to be that they provide more opportunities to practice the algorithm. Some may think this affords a benign and reasonable way for adults returning to school to learn math at their own pace, to keep track of where they are, and to feel a sense accomplishment from plowing through pages of a workbook. It is second rate because the mathematical demands of the world inhabited by adults are not sufficiently emphasized.