Probably for the same reason why Google keeps removing customizable features from Chrome. The more of them you have, the harder it is to test the product due to all kinds of unforeseen interactions between them. Someone probably said, "there's 10'000 lines of 30-year-old window positioning code we have to maintain, let's replace them with 1'000 lines of fresh window positioning code, then we'll have more leeway to add more Copilot features into the Start menu".

The more likely sounding conclusion is that everyone non-technical at Microsoft (sales, marketing, design, product management) uses macOS and assumes “look like macOS” is automatically good and they’ve never considered anything else.

There’s nothing about C++ which makes it “the only programming language which can draw things on the left”.