Obviously they could add it - it wouldn't be hard. Just because it doesn't have feature X doesn't mean it's bad. At that point it's a conscious decision to exclude that feature. Poor performing mobile sites? That is something they could for sure address, especially with the engineering power they have, I agree.
Twitter's been around for 10 years—and maybe it's just my perception—but I feel like the tech. media has never done anything but hate on them. It's well-founded criticism at times, sure, but at what point does it become a loop of negative attitudes simply based on pre-existing negative attitudes? That's just my take on the whole thing. I personally have some confidence in Twitter and have nothing poor to say about their product / direction.
It will change the artwork daily by default, but you can change the settings to change on each new tab.