I don't think it is so much that devs were greedy, but more along the lines of working within the limitations of the time. Programming tricks have advanced, memory is pretty much a non-issue (it was still quite costly when Nintendo launched the N64). Then there are the deadlines in place, especially for producing a cartridge game and hitting a sale date.

Plus, I think a lot of people that spent a lot of time on one piece of hardware get, well, bored. They want a challenge, they want to work on the next big thing. The newer generations are often the shiny object that has their attention so they go for jobs that focus there later in the life of the previous generation.

When we look at Altered Beast on the Sega Genesis, I would have laughed had you told me that a few years later we would have Streets of Rage 3 or Sonic 3 and how much they did.

Plus, here we are dealing with homebrew independent "I will work on it when I have time" developers. They are not under the same pressure to do things - and often the artwork is downgraded over being created from scratch so they can pull a Nintendo like with Donkey Kong Country.