Why start with flat paint on military aircraft and such?

Yes it does.  Sadly not all colors are available as a gloss so you're sort of stuck having to add the extra coats to make things glossy and then take them back to flat.

Flat paints go on easier and dry a lot more quickly than gloss paints.  If I paint a gloss coat, it has to sit for several hours and usually overnight before it is dry. On the other hand, if I paint with flat, it usually dries to the touch within an hour.  Even if I had all the colors I wanted in gloss, I would still use use flats for speed and convenience.

There is a company making military colors in gloss, xtracolor I think it is. I have heard they are really good paints. I think you can get them at www.squadron.com If not you might have to order them through Hannatts, but I'm sure shipping would be excessive to cross the pond!

Because I'm cheap!!!

 There is alot of detail painting that doesn't get the dull coat spray so I get the flat and use it as AB and brush paint.

Flat paints are easier to produce and "shade" (shading is the paint technology term for matching the color to a standard). As a general rule, they also have better hide (the ability to cover, or hide, the underlying color) than gloss paints. They are also generally cheaper to produce.

Gloss paints also usually have a higher binder to pigment ratio—more binder and less pigment. This is the reason they don't hide as well. Binder is usually the more expensive ingredient. As a general rule, gloss paints take longer to cure. The technical reasons for this are beyond the buffer capacity of this forum…