One idea is that companies don't want to make things that last because that is less profit for them. They are incentivized to sell things with shorter shelf lives that will need to be upgraded or replaced. Some companies like Apple go out of their way to make their electronics non-repairable, so people are forced to buy the latest and greatest.
In the stock thread here on this forum we had a discussion back when Weber grills IPO'd that they were a poor investment because the grills are too reliable, and when they sell one they take one of their customers off the market for 20 years instead of being able to re-sell to them in 3-5 years like other grill companies.
And it was a good point as the stock performed terribly and they were I believe eventually bought out and made private again.
I think the answer to
@Bri 's question is pretty simple. For capitalism and the stock market to work, profits have to keep increasing. Profits can increase in two ways, raising prices and cutting costs. Companies eventually hit a limit on raising prices, and have to focus on cutting costs. Cutting costs creates lower quality products.
But the market by design is somewhat efficient in handling this with recession. When a company is new they may focus on good products and low cost because customer acquisition/retention is the key. Once they become established (and especially if they go public) they have to shift that focus to maximizing profits (higher prices, lower costs/quality), especially if the economy is strong and people don't push back on it. Eventually we hit recession, and companies are forced to switch back to customer acquisition/retention mode by lowering prices relative to quality because customers are cutting spending, then we cycle back to profit maximizing, and so on.
The problem is we've used policy to skip the last two recessions, so we never haven't had that period where customer acquisition/retention is important in the last 15 years. So companies have continued to push profits, which has caused the market and multiples to shoot up, which means they have to maximize profits even more, and so on.
The other problem with that lack of recession is that we've created these incredibly high expectations of what life should be like, and adapted to a higher quality of living. So now we have people "struggling" while living a life that in the past would have been considered ultimate luxury. And that's not meant to belittle anyone's struggles because of course there are people out there really struggling, but as a whole we complain about things that in the past we would never have dreamed of thinking we deserved or had a shot at. It's been so long since the economy truly sucked, that we've forgotten what a truly sucky economy is (and a lot of young adults have never actually seen one!), and now we get upset when things drop from "really really good" to just "really good".