Because that's what capitalism selects for. Making profit is a singular selection function for companies, with any social good being strictly incidentally. So, the slimiest companies that cheat, scam, pollute, and do anything they can to externalize their costs end up dominating the market over time. Early on, there is incentive to make a better product to get a customer base and brand recognition. But competition means that some companies win and other lose, which means winners get bigger and harder to compete with over time. Once they get big enough, they no longer have to worry about making a good product. They can just muscle out or buy up new competitors. That's the monopoly stage, and at that point enshittification becomes the name of the game.
Destin from Smarter Every Day just did a good show on the problem incidentally https://www.youtube.com/watch?v=vS6HEes8daw