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A BUNCH of very good reasons.

1. Energy efficiency. Home canning is all about thrift. Heating glass jars with the hot air of an oven is less energy efficient than using a bath of water. Sure, it takes a wackton of energy to get the pot boiling in the first place, but once you've got it boiling you can do batch after batch after batch. When I'm done, I start a load of bath towels and pour in the (slightly cooled, no longer boiling) hot water. Because I paid for the water and the energy to heat it. With an oven, you open the door to take one batch out, all that heat comes spilling out.

2. It doesn't achieve higher temps inside the jar. Sure, you could put a canning jar in a 400 degree oven. Don't. But if you did, the water inside the jar would boil at 100C and get no hotter. So this wouldn't achieve anything more than water bath canning would. You don't want a rolling, bubbling boil inside the jar, that might push the liquid or solid contents of the jar out through the rim causing the seal to fail. Even if you did get the food sterilized, it would get recontaminated during storage.

3. High temperatures could damage the jars or lids. The lids, like all metal food packaging, has a plastic liner. Melting that onto my nice preserved food sounds like fun. For the sake of energy efficiency and thermal stress on the jars, the jars are typically preheated and packed with food still hot from cooking, but by the time you're ready to process them they may have cooled to, say, 60-70C? 140-160F or so? Putting them in a 400 degree oven might thermal shock and crack the glass.

4. Precise, foolproof, repeatable heating. I'm a 21st century suburbanite with a modern electric oven with digital thermostat, I STILL have to rotate a pizza halfway through baking because the back-left corner is the hottest. If you underheat a jar, you're gonna die of botulism this winter. Imagine trying it in some landlord special, or a 1950's Wife-O-Mat. Meanwhile, I can heat a batch of jars all to precisely 100C with nothing more sophisticated than a big pot full of water over a wood fire. Since I know the PRECISE temperature the jars are exposed to, I know EXACTLY how fast heat is soaking through them, and thus I know when the ENTIRE contents of the jar are heated to a safe temperature, and it works no matter what the weather is like.

For the same reasons mentioned above. Temperature isn't the problem. Without pressure the water turns into steams instead of boiling before it reaches a high enough temperature to kill the spores.