The reason is that generally citrus are not worth growing from seed, better to buy trees from a nursery. This is not necessarily the case with rare varieties, since nurseries may not carry them. One positive aspect about seeds is they are very cheap and easy to ship in the mail, but a negative aspect about them is that citrus seeds tend to have a short shelf life and would need to be shipped out soon after the fruits are harvested. This makes the normal "buy it when I see it" business model very difficult.
Something many people may not realize, if you grow from seed you're going to be waiting a very long time for fruit. This is due to two reasons. First, the seed takes time to grow to a small sized plant, the type you'd find at a nursery, and young seedlings can be more vulnerable to temperature and environmental conditions. The second reason is that if a fruit tree is not grafted onto different rootstock, the tree will keep growing and growing and get quite a big size before it starts producing fruit. The dwarfing effect of a rootstock diverts the tree's energy to fruit production rather than growth, earlier in the tree's lifespan. In addition, depending what citrus variety it is, there is a chance it may not grow true to seed, meaning the fruit characteristics may not be the same as the parent. That's what happens when genes genes get shuffled around. Most fruit growers find the risk is not worth tending to a tree and waiting many years.
