I think Tallie has it right. It's likely a trade-off between stressing the yeast with CO2 and having oxidized beer. I would add that degassing or adding O2 after fermentation has started also introduces a mostly unnecessary step, creating more work for the brewer and increasing the risk of contamination.
Depending on how early in fermentation we're talking about, O2 can actually be added to beer. From Yeast, by Chris White and Jamil Zainasheff, p 83:
If it is a very high-gravity wort, more than 1.092 (22 ºP), you must aerate with pure oxygen, as air will not pvodie a high enough level of dissolved oxygen. Unfortunately, that still might not be enough for beers higher than 1.083 (20 ºP) For high-gravity beers, adding a second dose of oxygen between 12 and 18 hours can help fermentation speed and attenuation. (O'Conner-Cox and Ingledew, 1990)
Here, we're not talking specifically about degassing, but it should be noted that oxygen is sometimes added to beer after fermentation has started.