As others said, the chip was more sophisticated than the 8088 itself, so it was fairly expensive. The original IBM PC (and pretty much all of the clones) came with an empty socket for the 8087. You could buy the chip and plug it in if you wanted the extra floating point performance. At the time, probably most people who bought it were using big spreadsheets, and it made using those much faster.

At that time, the idea of deliberately disabling features for market segmentation was seen as unseemly and an indicator of an illegal monopoly.