Using diode for reverse current protection
Such a diode would


add to the size of the chip and hence to the price
drop some voltage, hence the chip would have less or you would have to supply more


Nearly 99.99% of the chips are bought by professionals, to be included in a carefully designed circuit. If that circuit needs reverse-polarity protection such protection is added at that (circuit) level, in a way that is appropriate for the product (diode, fuse/crowbar, asymmetric connectors, etc). Putting such a protection in each and every chip would be bad engineering.

Or see it this way: chip are not designed and optimized for the careless hobbyist, but for the large-volume buyer. Such buyers generally do not want to pay (in price or performance) for things that are not needed in a well-engineered and well-produced product.