You can’t write code inside your browser because there are better environments for writing code for most people who want to write code, and enabling in-browser coding without it being a giant and regularly exploited security hole through which non-technical users would be exploited is nontrivial.
Yes, it would be very convenient for beginner programmers, but beginner programmers are a very small subset of browser users. (And a robust and even less-secure-against-”paste this command into your browser programming environment” attacks version would be good for more advanced programmers, but even worse for everyone else.) And having specific browsers targeting each of those environments would be possible, but negate (especially for beginner programmers) a significant part of the advantage of having the programming environment integrated in the browser in the first place.
Yes, it would be very convenient for beginner programmers, but beginner programmers are a very small subset of browser users. (And a robust and even less-secure-against-”paste this command into your browser programming environment” attacks version would be good for more advanced programmers, but even worse for everyone else.) And having specific browsers targeting each of those environments would be possible, but negate (especially for beginner programmers) a significant part of the advantage of having the programming environment integrated in the browser in the first place.