People are talking about the ease of locating plants to transplant rather than direct seed.
I want to make the case that there are some things that are easier and make lots more sense to grow from seed. Sure, tomatoes, eggplant and peppers make sense to buy plants if you don't want to start indoors earlier, and they produce in one cycle -- one plant, all season. Period.
Other vegetables and herbs make much more sense to direct seed -- they're just too easy, some don't like to be transplanted, and some only last part of the season and will need to be reseeded (or replanted when the season is late and there are none left!) Also, some things you just can't get at the market, such as zucchini blossoms!
Lettuce. You can buy a 6-pack but most it's easier to seed a row. They come up in a few days if you water them (they'll need that), then you pull some out to give them space, and voila! Your lettuce crop.
Arugula, spinach -- just too easy, same as lettuce, and you can put in now in northern climes where I am.
Mizuna -- a great tasty peppery green for salads and/or cooking.
Cilantro -- needs to be reseeded every few weeks because it goes to seed.
Dill -- ditto
Zucchini. No one will say they lack a green thumb once they stick 4 seeds in the ground and up come the sprouting plants in a few days. Also, the variety you can get by seeds is so much greater -- and tastier -- than the standard commercial variety. Why not plant a delicious Italian variety that bears a lot of male blossoms, and then stuff and fry them. Mmm good!
Cucumber -- easy as zucchini. And they resent transplanting.
easy. Plant a row and you'll have garden fresh greens all season.