1- Why don't you (meaning me) direct seed everything right into the ground?
Easy answer......I have too many things that destroy my emerging seedlings. Veggie seedlings must be far tastier than "weed" seedlings, because things often destroy my veggie seedlings before they get a chance to sprout true leaves. They get eaten by mice, rats, assorted birds including turkeys, slugs, cutworms, and a host of bugs. Believe me, I've really tried direct seeding everything. It surely would be a whole lot easier and faster. Prodded by one of the community garden volunteers, I tried planting an entire bed (3' x 22') with tomato seed and another with broccoli seed. We sowed three seeds in each spot....2 rows in the bed with seeds planted 24" apart. A lot of the seeds germinated just fine. By the following week, almost no seedlings were left. By two weeks, only one tomato and two broccoli seedlings survived. By week number three, all were gone except the one tomato seedling which had a severely damaged stem, most likely from a slug munching on it. It soon died.
2- Why do you direct seed beans and peas?
Because these don't get destroyed as readily as my other veggies. By planting excess, I end up with enough adult plants. I normally plant twice as many seeds as I hope to have as mature plants. I check the seedlings daily and watch for cutworm activity. When I spot some, I'll snuffle around the adjacent soil looking for the caterpillar and destroy it. I also look for weak or malformed seedlings, which I'll remove in order to leave more space for stronger seedlings to grow.
3- Why did my seeds ALL fail to germinate?
There could be a lot of different causes. Each would need to be considered, since I can't see your garden firsthand.
...old seed. Unless stored under ideal conditions, seeds normally have a lifespan during which they are viable, that is, will be alive and can germinate. Some veggie varieties have longer lifespans than others. Parsnips are notoriously short lived (1 year) while others may last several years.
...seed stored under poor conditions. Some veggie seed can survive poor conditions while others are real sensitive about it. If you've been tossing that seed packet into your desk drawer or workshop shelf, after a few months the seed may not be too viable. The constant temperature changes and humidity flucuations may do them in. Personally I find my best results with refrigerating my seed in a glass jar with a tight lid. By the way, freezing fresh seed may also kill it. Seed that has not had a chance to dry down correctly, plus many tropical seeds, can be killed by freezing.
...wrong germination conditions. Some seed requires light to germinate. Others don't. So if you planted a container with lettuce seed and placed it in a dark spot, it most likely won't sprout. Seeds also like a bit of moisture, but not too wet. I've seen gardeners who kept their sprouting pots too wet or too dry. Semi shade is good for seed sprouting. Letting them bake in the sun and dry out can kill emerging seedlings. Temperature can be important for some seeds. Bean seed requires 60° or above. Papaya wants higher temps. Sowing depth can also be a factor. Some seeds seem to sprout regardless of how deep they go (such as corn), while others are depth sensitive. Some, like tobacco, want to be in the surface.
4- Why did my seedlings die?
There could be a number of reasons,
...fungal disease called damping off. This is quite common with non-sterilized potting soil.
...too wet or too dry. Keeping the soil evenly moist gives best results. Totally drying out, even once, is a death knell for many seedlings.
...too much sun or not enough. Some veggies can take full sun while others want semi shade or shade. So, one needs to gear it to the veggie being grown.
...too much wind. Emerging seedlings are fragile. I protect mine from the full force of the wind.
...too much fertilizer too soon. Emerging seedlings don't need fertilizer right away. And they can't handle a strong dose.
What are some of the other failures I've had sprouting seeds?
...One of the cats deciding to sleep on the tray of emerging seedlings. Guess it must have been a soft, comfy spot. She crushed all the tender lettuce in the center of the tray.
...Trays getting knocked over and crash to the ground. Cats?
...A rat getting into the mini greenhouse and eating all the cabbage seedlings. Yum, full tummy!
...Farmer getting too busy and forgetting to water the seedling trays one day. Dumb mistake.
...Farmer being in a hurry, grabbed the first handy metal water can and watered the seedling trays. Water was scorching hot because the dark green can had been sitting in the sun all day. I cooked the seedlings.