An SSD is relatively very expensive per Gigabyte compared to a traditional HDD. Unless all consumer-level laptop maker's switched to SSD, those few makers that did switch would have to make their laptops considerably more expensive than their competitors who continue to fit an HDD. Not good for market share.
Consumers (most at least) would not be willing to pay that extra cost for less storage space when they can buy what may be more powerful laptop with a 1TB HDD for the same money or even less.
Buying your own SSD makes more sense because it will almost certainly cost you less than the laptop maker adds to the overall price of a laptop which already has one of the same capacity.