Because Omega did not make or designate it as a Seamaster. The word Seamaster was put on the dial by the redialer (refinisher) to increase the apparent value.

 

Imagine like with a BMW E36 318is. It's a 4 cylinder car with no balls and pretty ordinary handling, if you buy an M3 badge off eBay and slap on some wing mirrors it will look like an M3 but it's still just a 318is, similar deal here (well a seamaster isn't an M3 but it's still a case of changing the badge to convince people it's something it isn't)

 

So it's not a Seamaster, but a BMW? Now I'm confused.

 

Or a Mercedes Constellation.

 

IMO, not much. Movement looks like a mish-mosh of various Omega parts from different watches.

Whatever someone will pay for it.

I wouldn't buy it with your money, but that's me.
gatorcpa

 

The movement is most of the value in this watch, if you consider it as a collection of bits, the movement is worth a little the case a little the dial nothing, add it all up and you're under $200 in my estimation (anyone wanting to disagree feel free to but based on completed ebay listings under $200 is where I'd put it)

 

Its definitely a mens dress watch, back in the day mens full size was considered 34-36mm and extra large was 40mm-42mm while ladies was 25-28mm. Even today, mens dress watches (not sports watches) should only be 34-37mm, consider for example the Patek Calatrava line:

http://www.patek.com/contents/default/en/5120_1J_001.html

Considered the greatest dress watch maker on earth, their standard size is identical to the size of this watch you're asking about.