Hey Gigabyte,
There are certainly many laptops on the market that do come with a SSD or can be configured with one from the brand's website. As noted above, the reason where it is not in every laptop is due to cost. The price per GB is decreasing year over year, but they are still relatively expensive compared to regular hard drives.
They are generally excluded from budget and mainstream laptops to keep prices down to where most people can afford to purchase those laptops. SSDs are more abundant in laptops that are more expensive as well as thin and light.
If you are looking for a SSD in an inexpensive laptop, then you are not going to be able to find on. You would need to purchase the laptop and a SSD, then replace the HDD with the SSD yourself.