Just a guess, but doing what you describe to patent law would seriously and obviously harm the economy and isn't politically feasible, whereas insane copyright terms just give the assholes at Disney a license to print money, some of which finds its way back to the very corrupt bastards who extended the term in the first place.
Put another way, copyright as it is currently conceived degrades the creative space, but it isn't directly taking food out of people's mouths (it is, but in a less obvious way, and no one gives a shit about artists anyway).
It also means everyone knows patents are bullshit and collectively the companies don't want any of it but since we've got it established already and it's really hard to shut down the companies know they need to play ball yet do nothing in favor of patents.
Put another way, copyright as it is currently conceived degrades the creative space, but it isn't directly taking food out of people's mouths (it is, but in a less obvious way, and no one gives a shit about artists anyway).