Because this is a sport built on advertising. Look at the PR that Max generates for Redbull – that’s something that’s hard to quantify.
Hamilton is probably the biggest draw in the sport right now, but he won’t be around forever. Pitting him against someone less complacent could help build the reputation of a young driver. Bottas is great, but he’s never going to be a star. More drama means more eyes, more tweets, more clicks, and more time in the headlines.
You obviously don’t want to upset the balance so much that you self-sabotage, but there’s more to the equation than “it’s not broken”.