Because that tends to be the default for most images.
DPI is meaningless for most digital images nowadays, I suggest you ignore it. The only time it matters is if you send images to a brain-dead printer who insists on using the dpi instead just printing it at the size you request.
It really doesn't matter in most cases, but you can set up an export profile to set it to whatever you want.
Short answer: the dpi is set when you decide what size you're printing at.
Long Answer: Dpi means nothing in the digital world of your computer. There are no "inches" to have "dots per..." Size is measured in pixels. That's the same on your camera. It doesn't take 10 x 8 or 6 x 4 shots. It takes shots measured in megapixels. For instance 4,000 x 3,000 is a 12 megapixel camera.
Using that example, that shot from that camera has 12 million pixels. So that's how many "Dots" there are. To decide the ratio of dots per inch, you now need to decide the "inches" part. And that's printing. Print at 10 x 8 and the dpi will be 4,000/10 or about 400 dpi. At 6 x 4 then it's 4,000/6 or 660 dpi. Work the other way: Print at 300 dpi and the resulting image will be about 13 inches on the longer side.
So, your photo as a fixed number of pixels. Changing the dimensions of the print will vary the dpi, changing the dpi will vary the dimensions of the print.
For more see http://www.rideau-info.com/photos/mythdpi.html