Your ISP (or your router) can easily add their own JavaScript/HTML/ads on websites using http, it's likely that you are using a decent ISP that doesn't but is better if it's not even possible.
Public wifis are still a thing, many of them not well secured against Mitm attacks. Also compromised routers. So even if you trust your ISP (and their IT security and supply chain) there are still good reasons to want HTTPS everywhere
