They do neither of those things because, unlike you, they don't care about improving the user experience. They care about making money through any possible means.

1. Making the system lean means that you'd have to exclude all the ads, all the free tracking you can do to extract more money, all tie-ins with additional Microsoft services you could've done. Getting paid for the product key is just one step of many in the process of wringing their stack for every last droplet of money they can provide. If anything, it's beneficial to Microsoft to make Windows into a singular giant blob that amalgamates every Microsoft offering into one and pushes them as hard as possible. What are those mainstream customers going to do, not use Windows? Though of course, when using a lean system is a requirement for some business customers, MS will also offer a separate minimal version that can only be obtained through business licenses, just to avoid missing out on those few percent of the market.

2. Why make AI features opt-in? That would require your AI offerings to be alluring enough to motivate users to install the AI features on their own, and how many people will realistically want to install Copilot into Notepad or any other psychotic integrations MS came up with? No, you NEED for your investment to have returns, you need AI to succeed, so what you do is put it in the next update, and then progressively keep punching it down customers' throats enough (via pop-ups, colorful buttons, hardware Copilot keys, ads, integrating it into every piece of software - soon enough they'll probably start substituting regular features with AI ones) until it starts looking like the investment is paying off after all and the investors are happy.