> Why do developed nations need constant economic growth

They don't, technically - in fact it's not even possible to have unlimited economic growth in a world with finite resources. Eventually we won't be able to make more stuff with what's left on Earth, though hopefully that's a long, long way away. But just because we don't need growth doesn't mean it's not valuable:

• If your population is growing, like many nations are, your economy needs to grow along with it or you end up with less per person.
• You say people have enough stuff, but let's be real - very few people ever feel like they have enough stuff, or at least that more stuff would be undesirable. People want to progress and feel like they are advancing in their lives, and more or better stuff is one of the measuring sticks they use.
• A big part of the economy is people's perception of the economy. If the economy starts to grow less, people might be tempted to save more of their money, which means they are buying less stuff, which slows down the growth more, which encourages more hoarding, which slows down the economy more, etc. This is one of the ways recessions and depressions happen.