It depends upon the vegetable. Many gardeners in southern CA do plant another round of tomatoes in the fall! Some cultivars work better than others for that, though. The problem with many crops are that freezes are not the only factor in success. If you plant bell peppers before the soil warms up, for example, they will just sit and pout. Some plants don't produce without certain temperatures, others look to day lengths, etc. The foliage may look fine, but if they refuse to set fruit, there is little point in pushing them. Cold frames, or even poly covers, can help a gardener get a head start with the growing season. Or a greenhouse, if you have the $$$.

There are cool season veggies and warm season veggies. Tomatoes, for example, may live through our mild winters, but not set fruit because it is not warm enough. It's not just air temperature, but soil temperature that affects growth, and also length of days. Cool season vegetables should be in the ground now. I harvested a head of broccoli today along with a few peas, and lettuce is also doing well. Carrots are coming along as well as garlic and potatoes. Here is a planting list:

When to plant vegetables in Orange County

I've gotten large crops of cherry tomatoes in January that were volunteers in my yard, and so I do not know when they were planted (probably by birds), but for me, tomatoes do better in the winter than in the summer.

I live in the desert in SoCal where we getting freezing temps in the evenings,I planted a few tomatoes indoors and for the heck of it i decided to transplant one of them outside against my house i covered it with a Milk carton..

Just an aside to lgteacher - tomatoes most definitely will set fruit in winter in some areas. I live in northern San Diego county and I've got dozens of tomatoes ripening outside as I type this (they do take longer to ripen, but they will set fruit). My tomato plants themselves seem to do much better in the winter than the summer - less insect, disease and heat pressure behind it, I'm sure. Anyway, just pointing out that it may be worth giving things a try or two even if guides say otherwise as we have so many different micro climates that sometimes you can really push the envelope with what you plant and when.

In Southern California the best time to plant things is generally the Fall, so plants have a longer time to grow until the harsher Summer arrives next year.

I have have had in the bay area tomatoes live over the winter- and have tomatoes ripen very early- April. I didn't plant them. I guess some kind of compost market tomato. Taste wasn't the best..

btw- I ran into a article on the hardiest tomatoes that do taste good and grow all year in California.

Then I lost the webpage. Even the name of the Tomato was impossible for me to remember..some weird name. Google for it

Yes, tomatoes may live but not set fruit because of the temperatures, but when it's warm enough, they can set fruit in any month. It's not the norm, however. My holdovers look healthier now than in the summer, but I suspect that's because it's too cold for the usual pests. The fruit has been green for months. There are cool season tomatoes like Stupice and Manitoba that grow well in cooler temperatures.

I harvested broccoli last week and the peas are coming in. I also have a few out of season peppers, but the plants aren't growing like they will in the summer.

The original question was why not grow any vegetable at any time since they won't freeze. You can certainly experiment, but there are seasons during which certain things grow better than others.

Not to be a pain in the butt, but tomatoes ripening and setting fruit during winter IS the norm where I live. Just had BLT's for lunch yesterday and we've been having them in our salads all winter. It may not be the norm where you garden, but it is where I garden. There are such varied micro climates in SoCal that I would hesitate to give "don't do it" advice to someone 10 miles from me (which can be 10-15 degrees colder in the winter and hotter in the summer), much less in a different county, which is why I encourage experimentation to the OP. Learn what works where YOU live.